Well you definitely are on the right direction by looking here for the info on these machines. To thoroughly answer everything you want to know about the 900s and their quirks is a tall order however.
To start off, I would
HIGHLY recommend reading this thread, tons of great info for buyers covered here -
http://www.snowestonline.com/forum/showthread.php?t=112723
It is hard to recall all the tips right now as we are sliding into the off-season, but its fair to say that many of the trouble-spots for the 900s are not that unusual, its just that you need to take some extra time to check and setup the machines for the long haul. A few things that come to mind are the front radiator delete, ski change, spindle change, sway-bar delete and shock/spring upgrade, rider select delete, VENTING, clutch setup for your conditions (there is argument here on stock vs. other setups... seems to depend on rider and specific machine quirks), gearing it down, and things you can do to lighten the sled up some from stock... its pretty easy to take 35 lbs of these things with minor mods; can, airbox, wheels (if you don't ride any trails), etc, etc.
The best thing I could recommend is exactly what I did when I decided to look into a 900 two years ago, and that is READ READ READ!!! Since the 900 forum came to be, we have had some GREAT dedicated 900 setup discussions and you will find dozens of informational threads in the stickies at the top of the 900 page.
Until you have any single specific questions to post I'd recommend that you just take your time and do a thorough visual inspection on the machine. Pay close attention to the throttle body boots going to the motor - they have a tendancy to crack and can cause a burn-down if your motor goes too lean, and also check the condition of the oiler cable and that it retracts easily... it is a 2-piece setup from the factory and kinda funky... some last only a few hundred miles, and some (like mine) will go just fine for thousands of miles untroubled. If you do a compression test be sure the cylinders are within 10% of each other and read at least 100PSI each... anything less than that means you will be needing to do a top-end rebuild and have to pull the motor to do it on the 06s. With only 800 miles on the sled I think you would be pretty safe from anything major for quite a while IMO.
There are many of us who just love the 900s despite the extra care they sometimes require... they aren't necessarily the most powerful or lightest sleds out there, but the way they do make their power - especially the low end torque - is very addicting and great for running through the trees without having to rap it out all the time.
Sorry for the long post, hopefully that gives you something to start from though
Good luck with your decision
. Stupid y-pipe bolts. I need to find that other thread and get those lock washer things.