Broctoon

So you need a bike for transportation, not a hobby... yet you enjoy wrenching more than riding. Trying to figure out which it is. What did I miss? Oh, and there's a high likelihood anything you get will be stolen? Given your situation, I'll second the recommendation from a few posts up that you buy a cheap, solid, complete bike. If you still have some money and want to wrench on a bike for fun, you can gradually build up a second one. You're going to search for a frame and a collection of pretty specific parts, spend the time piecing it together, (once again) spend more money on all this than an assembled bike would cost, and then be really sad if it gets swiped. Get something that you have no time or emotion invested in, and when it's stolen you'll shrug if off.

I don't mean to discourage you. I've fixed up a few old bikes before. It was a good experience for me, because 1) I didn't need them, as I already owned several cars and a few other bikes, and 2) I had some disposable cash and free time. There was no time crunch or specific budget to stay within. You don't seem to have the same advantages currently. What Ironfish653 said above is true. A complete bike usually costs much less than the sum of its parts. Unless you have a stockpile of old parts on hand, or your co-op is really well stocked and easy to work with, a mid-range 1990s MTB will cost more to assemble piecemeal than it will ever be worth--even if you buy used parts. Think about it, just as used parts cost less than new retail, so do used bikes. Proceed with your plan if you're sure it's what will make you happy. I hope it does. Just proceed knowing what you're getting into.