You will need a case, power supply, cpu, ram, motherboard, monitor, operating system, office, desk, mouse, keyboard, electricity, hard drive, cd-rom, I like to have a chair.
initial cost will be about $1500 and will be about $15/month
For basic office use, the cheapest machine available today will do just fine. You can buy a preconfigured machine from Dell or any of the big retailers for less money than you can build one yourself.
Not that there aren't advantages to building one yourself and I always do, but it will cost more.
I would say you could look up the more recent Toms builds for their budget gaming machine and build something like that. It will handle all of your office tasks and if you want to game with it, it will do that too.