I would offer em 15, they won't take it but you can go up from there. Even though we love em Tbirds are not hot sellers. A turquoise one sat on the Ford dealer's lot near me for over 2 YEARS. 54K miles is not many miles, until you start comparing em to other Birds. I can think of a half dozen around me that have less than 10K miles on em, and those are the ones the so called collectors will be after.
I would also check to make sure EVERYTHING works as it is suppose to. Don't assume because some small electrical component doesn't work it is any easy fix - it could be the beginning of the dreaded
Front Electronic Module problem. When I purchase my 03 over Labor Day Weekend the
AC wasn't blowing super cold, but cool enough to work on Labor Day, I just assumed it needed a can of Freon. No it needed an effing
compressor. Check
windshield washer pump, make sure automatic lights come on and go off when suppose to, check
fog lamps,
radio, everything. If something is amiss - walk, no RUN away, and find another one. I love my Tbird but this thing has been a nightmare mechanically for me. Some of those small seemingly simple fixes are related to bigger problems on these Birds. Nothing simple to fix on these things,
parts are getting hard to find, etc..
Good luck! Not trying to talk you out of buying one, because sometime after a rough day at work, even in the winter it is nice to drop the top, turn the heat up, turn electric seats heaters on and go cruising, and it is all worth while.