Multi- user tower

[Brucethehunter]

So I live in the Caribbean and we tend to end up paying double if not triple for parts so I don't often build computers but I have been thinking about building a sort of 3 in 1 computer. 

Here's the thing I have to build a new cpu for myself and everyone's computers in the house are a little dated and showing signs that they would need replacing in the near future. So I been toying with the concept of build a more beastly CPU and multiple users 3 exactly they are as follows:

Parents user don't need a particularly powerful build as the mostly just use Facebook, Netflix and YouTube nothing to fancy so I could just assign 2 cores, 4 gb of memory, 100gb of SSD and a modest graphics card and that should be fine. 

My sister is mostly a emulated gamer and writer ideally she doesn't need any massive computing power either so I'm thinking 4 cores, 8gb of memory , 100gb SSD and 500gb hdd and another modest to maybe lower end of high range graphics card.

Finally myself I'm not a massive gamer or anything but I may want to stream gameplay from my switch thru the capture card but their are a few games I wouldn't mind running on PC either so I'm thinking maybe 8 cores with  12 gb of ram 200 gb SSD and a 2tb hdd and like a GTX 1070 or 1080. 

Now from what I understand the system will basically be almost always on I'm thinking it maybe better to use a xeon processor with registered ecc ram if I could cannibalize a server of some kind to save on cost that would be ideal but I'm not married to it so if someone could suggest a parts list that would be ideal would be a great help.