I agree with others saying you might only need one computer. $2500 buys you mostly newer stuff, except maybe the GPUs. But one machine saves a lot of headaches. You'd go with multiple machines if you wanted some high availability or redundancy but you'd need to set that all up (plus a single failure is a single failure). Plus if you go with windows, you'd need multiple licenses (which is no big deal, maybe $20 a pop).

In this case, Id say it's best to stay away from hypervisors being a small business since you don't want to devote a lot of time maintaining your system; and instead of running a complicated storage setup use a a mix of fast NVMe drives and large 5 year warranty drives and a separate NAS located elsewhere in your home (or even better, pay for a cloud based backup solution) that does INCREMENTAL backups once a month, once a week, and once a day. That saves on how much bandwidth you use but has enough backups where a daily oopsie can be reversed and you have an old enough backup to shrug off a ransome ware attack (once you delete everything and implement a more hardened setup). If you already pay for Microsoft office, you have 1 TB OneDrive storage that you can use as a free option, depending on how big your critical files are.

Sounds like you have windows but it's also dependent on what your software requires (access to opengl, access to GPU, etc.) that might make sharing the one computer much more complicated. Assuming it's simple (GPU and opengl acceleration) RDP is a good choice, it's sturdy and built in and doesn't require any command like stuff. Note that windows pro only allows 1 user to be logged in at a time, you'll need to use something called rdpwrap to defeat that. Conversely you can pay a lot of money for windows server and have that unlocked - at that point, I'd look at running Ubuntu.

5he other part of the conversation is how they will remote into your home. I highly recommend setting up a tunnel and only giving them access to their computers. The easiest way to do this is to buy a router with a tailscale client built in, put all the computers they need behind that router, and then have them install tailscale on their own computers. When you are done with the intern, you can easily revoke they access through the tailscale web portal.

Lastly, your Internet provider needs to be up to snuff. I would say 100 mbit up is reasonable of all five people are going to be in there at the same time. That translates to 80 mbit actual performance, 20 for your household use, and 60/5 = 15 mbit for their rdp which is more than enough. I have 10 mbit up in one of my locations and it sucks.

There are tons of other, more complicated and more expensive/cheaper ways to do this.