Its worth getting if thw price works and it fits. Ypu dont know/mwntion either.

90s carbon- look at it carefully and make sure nothing is chipped or cracking.
Make sure all the components work as they should, except the obvious derailleur.

A new front derailleur is $10-50 depending on what type you need. If the shop sells it and installs it, id plan on $50.
Parts add up fast so have a max budget and decide if the bike plus any repairs or updates exceeds the budget.