Home Servers for BOINC, AI Dev/testing, Website etc

I recently got my hands on a really nice used server, and I have been developing my website on it, along with using it as my mess around platform, and I have apparently been bit by a bug.

That combined with being introduced to scientific computing like BOINC, I would like to build a system (or several quite honestly) that I can give to BOINC when I am not using it. (and that fit in the server rack that I got with my old system)

When I am using it, I want to be able to use it as a learning platform for AI dev, as that is one of the things that I want to dive into more, and multi-server programming (why I want multiple physical machines). I also want to be able to use it for rendering if I ever get off my but and actually make videos. (IE not a priority)

Unfortunately, I have never really parted together a server before. Im not looking for super cheap, but im also not looking for brand new either (IE, im prob going to be buying off of ebay). I suspect that I am going to need something with decent GPU support, and the GPUs to go with it, but I have no idea what to start looking for.

Its not like I will be putting it together tomorrow, but some help on what I might want to pick up in the future/start looking for would be appreciated.