The most popular model on the second hand market is the 530d GT, thanks to its blend of punchy performance and reasonable running costs, but there aren't likely to be many 5 Series GT models around - especially compared to the 5-Series Touring or X5 - as it's quite a niche car. The SE is the standard model but comes very well equipped as standard. Most people buying new will add on plenty of options, that won't necessarily add much value when they sell, so you could get a very generously kitted out car.
Key extras to look out for are sat nav (there are two versions to choose from), adaptive cruise control (which maintains a set distance between you and the car in front) and the head-up display.
The rarity of the 5 Series GT means that any car for sale used is likely to generate interest. However, many buyers might not be familiar with the GT or what it's supposed to be. Advertise it like BMW does - as the best of both worlds between an X5 and a 5-Series, and you should have few problems selling. Buyers will prefer the cheaper to run diesel, but the 550i has plenty of appeal for those after something a little more flash.