$800 can build a pretty nice PC. That's usually what I spend every 5-6 years for an updated one for myself. I usually reuse monitors, keyboard, sometimes the case, etc.
Unless others know a better place to order parts, I get everything at micro center. Price is usually better than anywhere I look online.
These days building a PC from parts is very easy, most things plug and play, so not a lot of configuration for basic use.