I saw the ad online:
http://www.classiccarsglobal.com/ad.asp?ad=001329
Looks like a really nice ride and the reputation of Unique is outstanding. My concerns on a car that was built in '91 - and I'm going through this with my CCX '95 build - is that you have a fair chunk of upgrades to do, hoses, belts, fluids, etc. What is the guesstimate of what you need to do to make it suit you / get road worthy? Probably $1K on the low side, $5K on the upper.
How long has the dealer had it? Since it's a dealer you have to figure they took it in on trade and gave someone $15K, listed it at $29K now. It is the middle of winter with probably nothing moving... especially a convertible without a heater. I'd start low, pointing out the objects you need to address, and offer $19K or $20K. Worst case is that you end up back where you were originally thinking but my guess is they'll jump at any reasonable offer.
Good Luck!