sounds like work you could do yourself if you're even halfway handy. The front derailleur problem might be as simple as a limit screw turned too far in, for instance by somebody who thought granny gears were sissy and locked out the small chainring. If that's the case (or the reverse, for the big ring), it's a 10-second fix with a screwdriver. You could pull and lube the shifter cables for free, or replace them for a few dollars ($5 to $8, I imagine). I can't figure out what you're saying about the seat post, but my guess is that there's some way to adjust/secure it that you're just missing. If not, a new seatpost can be as cheap as $10 or so on sale. Brakes will improve dramatically with aftermarket pads, but that's $15 you might not want to spend. Cleaning the pads and rims and lubing the cables so they work smoothly might help a lot. Sand the pads lightly to remove the glaze if necessary.
You don't mention what model DB it is, and that would help with the decision. Some of the low-end ones (I think Sorrento may have been one; can't remember) were pretty basic. But with a couple of hours work and not much money, you could have a bike that runs fine, won't let you down and that nobody wants to steal. That's not a bad thing.