FBinNY 

I can't say whether investing the time and expense into your old bike makes sense, that's strictly your call. To help decide, figure how much the wheel will involve, then what other work would be needed, and compare what a comparable bike would cost. Don't forget to consider that you're comparing used to new and make the allowance for it.

As for how hard is wheel building, it's one of those things that's easy --- if you know how. It isn't hard to learn the basics from a tutorial (I suggest multiple tutorials to get a good sense of it), and lacing won't be difficult. Tightening and aligning is a bit trickier, but if you're building a classic 3 cross 32 or 36 spoke wheel with a fairly stout (approx 500grams) rim it won't be a bear. Like all newbies you'll stumble and fall into a few unexpected traps, but it won't kill you. The key is to be patient and work by degrees.

Much is made of getting even tension, and that is very important, and in case it isn't mentioned in the tutorial here's the secret. You don't want to try to get the tension even at the end, you want to keep it even all the way through. Likewise you want to get the rim spinning true fairly early on, and keep it true as you progressively and fairly evenly add tension.

Here's where some skill and experience come in. If you start aligning it too much early on you'll lose your even tension, but if you wait too long it'll require major tension imbalance to force it into true. It's a judgement call, so try to keep it reasonably aligned, but don't let spokes have more than one turn of the nipple with respect to each other. Also focus on radial mis-alignment first because this is hardest to correct later on, and the wobble can be brought into check as you move along.

best of luck as you go along, let us know how it works out.