If your tight on money, building a system will always come out on top in the long term, specifically in terms of upgrade paths to keep up with the times.
If you're actually strapped for cash, buying a used system will always come on top. If you're patient, watch goodwill and other spots for home-built systems that have new enough components to have an upgrade path in the future.
Why "back" of course!! I hear babies got it, so start with them.