I don't really have much money to spare, and this is still just an idea I've been wanting to put to action for a very long time now (4 years).

I've looked into conversion kits and hub motors, but they go upwards of $200 partly because they include a completely new back wheel. Although buying a kit is the easiest option, I'd prefer buying the parts myself because I am a stubborn frugal kid.

The bells and whistles (pun totally intended) are out of the question; I just want it to "work." For example, if there is any need of a screen, I'll just get a small Arduino for it with an LCD display; and regenerative braking on downhill is something I will take care of later. I just want a motor on the back wheel with a throttle control while keeping the pedals. Obviously I am not asking you to do the research for me, but does anyone have experience or tips in doing conversions?

Converting my bike to electric will almost instantly give me a reason to go outside. No seriously, the one reason I've never wanted to ride my bike is because the terrain where I live is incredibly steep, so steep that on one side of my house there is a 35ft drop between the window and the ground, and on the other, a driveway that must go downhill to the garage.

Well, the hardest (and most expensive) part of all this is a good battery. It'll need to be rechargeable, and I don't think rechargeable AA's will do the trick here. You could buy a bunch of Li-Poly (Lithium polymer) batteries meant for hobby aircraft and wire them together, or you could spend a bunch of money getting a battery made for electric bike conversions. Something like this would help you. Not cheap, but you could then buy or make a electric engine for much less and make your own electric harness. Also, Li-Poly batteries really won't supply the juice needed to get up even a small incline. You'd need a really good electric motor as well. And about regenerative braking... It happens automatically when you coast downhill. Just about all electric batteries are wired to accept that flow back to them, so yeah, you'd be good on that score.