But it may be worth checking a few other things first, like belts, connections/leads; is the battery OK, etc. Check across the battery leads with a voltmeter also, and see what you get. (Or get a garage to do it). Unless you've been totally screwed over by the PO, I think it would be difficult to sell a car without a functioning alternator as it would be picked up on test drives.
Changing the alternator is not too difficult job. If you eventually do go down that route it would be worth upgrading to the 55amp alternator if you're so minded and don't already have one. (Not sure what the 250 had). I seem to recall that there are other units that could be used. (A Saab one, Naj?? He'll know that sort of thing and hopefully he'll be along soon!!)