I have an 08 2900MK. We looked at the 07's and I actually think the 07 may have some advantages over an 08. The 07's have a aluminum floor joices whereas the 08 went to wood. The 07 had a few other extras ( standard) outside wash ports and a few other internal options no longer offered in the 08's.
As far as reason why someone would trade it in after a year, who knows? Most the time units are flipped this early is due to financial related reasons. ( got in over their heads). Just be safe, I would do a very good ( detailed) PDI and look for any and all signs the unit may have been in a wreck or received major physical damage that needed extensive repairs ( e.g. insurance buy back unit). Look over the frame ( have dealer drop the underbelly cover) and axles very closely for any signs of repair. Look at the tires to see if they are wearing evenly. Also, if the dealer will give you at least a one year basic structual warranty, that should help ease your mind.
Overall, unless the unit was wrecked, the 2900MK is an excellent floorplan and is made very well. I have never needed to use my slider (ever) even in very tight situations, I still had plenty of clearance. I'm not sure I would spend the money on a slider if I was to do it again. If you do get a slider, you don't need an expensive automatic one for the few times ( if ever) you'll need it. I recommend you save the money and get a non-slider and take the savings to buy the best ( non-mainland china) tires your can afford!