you can get a basic desktop chassis, 4Gb memory, 500Gb disk space, mouse and keyboard for under $280. This is an example from
BestBuy. You might be able to use that with your existing monitor or you could get a new monitor for under $200.
If you ONLY use a computer at your desk and you have space, a system like the one above would be my suggestion.
However, if you think you'll be doing any TRAVELING with a need for a computer (like paying bills on the road), then I very much like the idea of a laptop. This would be a lower cost laptop that I think would meet your needs:
Best Buy LaptopI'm not recommending Best Buy as a store - just using their catalog as an example of systems to consider. One advantage is their no-hassle return and service intake at stores throughout the US. WalMart, OfficeMax, etc are other great possibilities. Since I'm technically knowledgeable, I buy my computers (primarily HP desktops and laptops) directly from the vendor website and have always had great luck.