From my experience, I'd rather an 07 Rev with an updated motor over an 08 XP all day long. Easier sled to ride and needs less mods to ride well than an early XP. A sled like what Cubby mentioned would be a great buy IMO.....and will still give 2012/2013 iron a hard time in many conditions. Gotta have the newer motor though....I can't imagine there are a lot of original 2007 motors still intact, as most piled up the rod bearings.
M8's were slugs with awful suspensions until 2010. Dragons rode well if you can find one that's had the motor re-done by someone reputable. XP's rode like complete crap until 2011ish when they started getting better. Some guys think they're great, but there aren't 1000 threads on how to improve crappy XP handling characteristics for no reason. Same as Dragon top-ends....some work great, but again, the stereotype is there for a reason.
Just one mans opinion.