There have been times in history when an argument could be made for either Mac or PC being the superior choice for a recording studio. Today, you can pick whatever you're more comfortable with. It really doesn't matter what you choose anymore. Mac used to be known as the "industry standard for professional studios", but PC really got their shit together and a lot of pros run successful businesses on PC's. I've used both over the last 25 years and I can truly tell you that you should pick what you already know. If you plan on interning at a pro studio that only uses Mac and all you've ever used was PC, make sure to take a crash course on Mac's so you're not incompetent. If you do pick Mac, just know that many third party software developers concentrate on designing their products for PC first and designing for Mac second. Not every piece of software on the market is compatible with Mac. Just make sure the Operating System you choose is compatible with the DAW you pick. Mac is still a good choice, there are plenty of software developers out there designing with Mac in mind.
BUY OR BUILD YOUR PC
Unless you or your buddy know how to build a PC, just buy one with the specs you need. I'd recommend a CPU with a minimum of 4 cores and at least 3.0 ghz speed. You'll want at least 8 GB of DDR4 Ram and decent audio and video cards. You'll want one SSD for your operating system, one HD for your projects, one HD for your libraries, and maybe an extra HD for personal use. If you plan on using large virtual instrument libraries like drums, strings, and orchestras, I recommend at least a 4TB HD for your libraries.
From the 1980's to the 2010's Pro Tools was the industry standard. It's still the industry standard, but just barely. The number two selling DAW is Cubase and Pro Tools only fractionally beats them out for market share. If you plan on working or interning in a commercial studio, it helps to learn Pro Tools. If you plan on working alone, pick whatever DAW that works best for you. No single DAW sounds better than another, but they're all just a little different. Some say the beat detection in Pro Tools is the best but the MIDI programming in Cubase is superior. Or maybe you plan on being a live EDM artist so picking Ableton Live is a no-brainer. And some prefer Logic because they're working on a Mac and they know it just works. In the end, it really doesn't matter.
PICKING AN INTERFACE
This really comes down to how many inputs you need for your studio. If you only plan to be a singer/songwriter and the most inputs you'll ever need is one for a vocal mic and one for a guitar, then pick any two channel interface that fits your needs. If you plan on recording drums, pick an 8-16 channel interface. If you plan on recording a full orchestra, pick an interface that has the right kind of connectivity to get you to the number of channels needed to get the job done. Aside from inputs, choosing an interface with the right types of connections is also important. Perhaps you'll be working with MIDI instruments or running additional preamps that use ADAT or need to patch in a Kemper Profiler with SPID/F. Additionally, choosing an interface with great routing software can be a real game changer. I highly recommend watching unboxing or how-to videos before deciding on an interface.
HOW TO MONITOR
If you live in an apartment, this is a no-brainer. Headphones are your only choice. In that case, I strongly recommend a pro set of open-backed headphones with a flat EQ response. And if you plan on tracking multiple artists at once, you'll need a headphone amp to power multiple sets of headphones. If being quiet isn't a concern, a great set of studio monitors are hard to beat. Be careful though, studio monitors don't sound great in an untreated room. In a pinch, you can use large objects like couches and beds or even blankets and carpeting to treat your room. Also, if you're planning on collaborating with other artists in your studio, monitors are a must. Nothing beats gathering around a set of speakers, cranking them up, and enjoying your music together.
UPGRADED THE PREAMPS
A microphone preamp takes the weak analog mic signal and boosts it to a level that's usable. Most consumer grade preamps are built into the interfaces these days and they get the job done just fine. For those looking for a high-quality signal chain should consider purchasing additional high-quality preamps. What makes a preamp high quality? Depends on what you're looking for. People say that API's have the best transient response on drums. They also say that certain preamps add a certain kind of desirable "color" to the sound. Some prefer transistors, some prefer tubes, and some just need additional workhorse preamps that are "clean". There's so many different reasons for choosing different preamps out there, take a look at some videos comparing the different types.
THE RIGHT MICROPHONES
This may be one of the most subjective parts to building a studio. Whether you're picking microphones for drums, guitar, piano, or vocals, the source has just as much to do with the final sound as the microphone itself. For example, one mic that works great for screaming male vocals probably sounds like garbage on a piano. A mic that sounds great on a snare drum would sound terrible on a soft female vocal. Check out some comparison articles or videos to help guide you to the correct mic to fit your needs. Yes, expensive mics usually sound better than their cheaper counterparts but that doesn't explain why the Shure SM57 (a $100 microphone) is still the industry standard for guitar cabinets after more than 40 years.
TO SIM OR NOT TO SIM
Simulated guitar tones were complete garbage until the late 2010's. Today, they sound better than ever and would probably surprise you how many professional records they're being used on. Sim's aren't for everybody but if you can't fire up a full guitar rig in your apartment, amp sims are a must have. I also prefer to use them as a tool while tracking just to keep things simple and moving along. You can always go back and reamp the guitars later if you're capturing a quality DI. Speaking of guitar DI, don't use the "instrument" input on your interface for recording. Always use a high quality DI box to get the conversion done properly between your guitar and interface.
KEEP IT SIMPLE
There are so many other toys out there to buy for a studio including outboard gear like compressors and limiters or VST libraries for drums or strings and even fader controllers for people planning on mixing music professionally for clients. Don't worry, by the time you get done buying everything else already mentioned plus all the necessary cabling, stands, and peripherals, you won't have any left over money for these extra bells and whistles anyway. In retrospect, I'm glad I was poor when I started. It helped me really learn what I had before randomly buying shit I didn't really need. I had a laptop, a DAW, an interface, headphones, and a microphone. Believe me, I got better at recording and mixing in a hurry using so few tools at my disposal. If I had spent thousands on gear and software I'd still be buried in instruction manuals and tinkering around with shit that doesn't even matter. Start small, read every manual, get good with what you got, then build out from there. Besides, you'll make better purchases once you truly understand your limitations and how to solve them.