Don't do it... a wing keel is complete disaster... 
Actually, wing keels are just fine, but if you do go aground, it is a bit harder to get the boat off, since you can't heel to reduce your draft.

Ask SailorMitch, as IIRC, he's a wing keel fanatic.
As for what to offer, I couldn't say, since I don't know the boat, and don't know the market you're in. Do you like the boat??? If so, offer $18,000 or so. While that sounds a bit high to me for a 28' boat that is 18 years old, if the boat is really as clean as you say, it is probably worth at least that much. It isn't what they're asking, but it does give you some bargaining room, for whatever turns up on the survey and to come up in price a bit.
BTW, I'd be a bit cautious, since, the boat is nearly 20 years old, and may be in need of some major refitting, like having the standing
rigging replaced. It may not, but at 18 years old, the standing
rigging is probably getting there. You'll definitely want a good survey done. Separate
rigging and engine surveys may be worthwhile too.