Would find out how much your likely to get for a trade-in first before putting any money into it.
If it's a 26" old FS bike, it's going to be pretty much worthless, if it has the Brain system, even less so, you may find that just replacing the crank with a new one may cost more/be similar to what you get for the trade in (used is a nice idea, but whatever you do, you will have the complication of the BB, as it could be one of 4-5 different systems, hard to tell from just the model of the bike, as to what you have).