I second the roku, they are economically priced, they perform very well, minimal ads. My friends and family use them to stream from my jellyfin server over the internet to their homes.

Well, openERP or openbravo were what I would have recommended ten years ago, but due to their commercialization aren't really relevant any longer. If I personally was setting this up for myself I would probably use redmine and a plugin that gives redmine the invoice functionality. However I wouldn't call it simple for a first timer to pull off, but if redmine is mastered you will find very extensible and customizable to any particular project's needs.

Lol, I really get that vibe to, but it is very interesting :P