IMHO, I would buy the most stock Buell you can find, especially for my first Buell. More than 8000 miles but less than 30k miles is a mileage sweet spot for the least amount of problems for its age.
The '08-up thunderstorm engines have some upgrades and a slight price premium, but I have not had any major issue with the earlier bikes.
They are pretty legendary for dependability, why mess with that? Are you going to drag race it?