Go fixed. It's fun. Seriously.

Don't worry about hills, dude. You will get stronger mashing up them and no one has died (to my knowledge) dismounting and walking the thing to the top. Personally, I feel like each sizable hill that I get over is a little personal achievement. It's also loads of fun going down the hill when you get to the top.

I say the cheapest option is to buy complete. I've never done a build from the ground up though, and I could see how that would be more rewarding.