RZR's are a blast!!
Don't get the original/basic suspension rzr, the S suspension is much better with or without fox shocks.
I understand that in some places you are limited by width and the narrow basic suspension may be the only option. That sucks but at least you still have an option.
2011's are available with or without fox shocks depending on if you want to cut costs. The fox's are worth the extra money unless you have other plans like elkas, etc. I'm not sure what they change for 2012 on shock options.
2011 rzr rear axles bars are were changed. They are even weaker than the past axles, twist off pretty easy right next to the CV joint. Easy fix, gorilla makes great axles. SuperATV has the Rhino axles which are better than stock but not even close to as stout as the Gorilla's. You probably never break the stock axles in the dunes but anywhere else and it's a good chance you will twist them off.
2008-2009 are notorious for blowing front diff's when you get them in a bind. Poor cage design that was updated in later models.
As far as motor building, those little 800 polaris motors just don't make HP compared to the V-twins like the 850 polaris, canam, kawasaki, etc. that are on the market. Might as well leave it stock. If you do cams, headwork, pistons, etc and you will sink a lot of money and still be lacking HP! I'm just not a fan of these motors for performance but they are reliable.
I have a guy that I race ATV's with who has a turbo RZR. It has a little GT14 turbo with an intercooler and seperate injector/controller. It's a beast. Much easier to make HP with the turbo than complete motor builds on these engines. Only thing is the stock head studs and stock crank will only take around 7psi boost (some more, some are less).
The 900 RZR is night and day different, if you hav the cash I would much rather see you drive away in one of those. The new 850 engine design has definitely raised the performance bar for Polaris.
You can always add power steering later, all of the RZR models are the same. SuperATV makes a bolt on kit for around $600 (I think). I won't own a SxS without power steering, just too convenient when you have a beer in your hand!!
On a side note:
The new Arctic Cat Wildcat SxS should be hitting the ground shortly. I know a couple of people who will have factory demo's to thrash and abuse to see if engineering did their part. Our clutch guy is getting one, he was a big push on the new clutch designs that will be offered from the factory by Arctic Cat.
http://www.wildcatforums.net/forum/.../137-17-new-arcticcat-wildcat-pics-enjoy.html
I'm still sitting on the fence about spending that kind of money. On the other hand, nothing fun is cheap. I'm leaning towards new because of the ease of financing / better rates. Plus I won't have to worry about the cost of fixing anything right away.