Uhm, not sure if you are aware the cost of the type of computer with the power to do this. You're talking about a computer that is barest minimum $2,000, and best bet $5,000+
A high powered professional gaming rig can cost $10k or more. You mention cheap start up, but at same time want multi-input pc... makes me think you've not researched to find out the price range of gaming rigs. They are not cheap.
Most people on YouTube who are playing on a gaming pc, did not start out to play games on youtube, but rather were adults, with high income careers, and already had the equipment, and were already heavy duty gamers for several years before they realized youtube existed.
It's very rare for someone to buy a gaming rig because they think they might possibly, maybe, think they might enjoy putting games on youtube. You are talking about a piece of equipment that not many people can afford to buy, because it costs the prices of buying a car or putting a downpayment on a house.
I say this because reading your posts, you sound very young and seem to be unaware of the cost of the equipment you are asking about. It's not likely you're parents can afford to buy you a gaming rig and a teen or college student likely will have to save for several years before they can afford one on their own.
You are talking about something that is a VERY BIG financial investment.
Certainly well worth buying if you want to play the heavy system load games like Witcher 3 or Dark Souls, but if you are not 100% sure you like gaming (which is what your posts seem to indicate) it might be best to not invest in something that expensive until after you've been gaming long enough to be certain you like it enough to spend that amount of money on a rig.
Keep in mind the average youtuber takes 3 years to earn their first $100, so you likely will not see a "return on your investment" in less then 5 years, if you ever see any income from it at all.
Youtube gaming, like any other type of gaming is a hobby first and foremost. I think a lot of people forget that.
I mean, I have a high powered gaming rig that literally can play every game out there, the system specs are that high, that fast, and that overpowered. It'll be several years before new release games catch up to my pc. But I'm also a retired career woman, who plays games several hours a day, only once in a while recording one to put up on youtube. I do youtube to share gameplays with my facebook friends, I'm not looking to have a career from it. I'm using a rig I have because, you need a rig like this to play the type of graphic heavy, high memory load rpg games I play. It's just standard equipment if you are gonna play stff like Witcher 3 and Dark Souls.
So, I'm reading your questions here and I'm not sure your age or your budget, but you sound like a group of teens relying on your parents to buy you gaming gear because you thinking gaming might be fun and you even admit you've had to go through the trouble of convincing your friends gaming will be fun to do, meaning they aren't really on board to gaming, they are just doing it because peers do what their peers tell them to.
I'm really not feeling the vibe that you are going to be into big scale pc gaming for anything longer than a fad, and I really think investing in a heavy duty gaming rig may not be your best option right from the get go.
If you are worrying both about budget and about keeping your friends interested in gaming with you (an indication that you don't like solo play gaming and will drop gaming fast once your friends drop out) I really don't think you should be looking into spending $2k to $10k on something you may soon find you won't use once your friends find the next fad hobby to try.
I would really recommend you be someone die hard devoted to solo gaming on your own, without relying on your friends to inspire you to play games, before you start looking into a pc with the system capable of supporting multi-inputs. You really are talking about a very expensive machine that it sound like you can not afford right now.
It would be different if you were already a solo gamer with a gaming rig and were looking to upgrade it for multi-players, but that's not what your posts indicate. Upgrading cheaply is one thing... setting up a high powered pc set up from scratch, is not cheap and can cost what a lot of people consider to be a full year's income.
Based on your posts, I think a gaming rig is something for you to look into after you have a bigger budget to work with. Something you definitely should look into if you want to turn gaming into a career as well, but, may not be the best option for you right now.