state trails are supposed to be 50" or under, but all of the ones I have been on do not have any way to measure, and you could drive a damn truck down them...so the RZR S will fit just fine (at least on the ones I have been on). With the stock wheels on, the S is about 60" wide...the old RZR with the new wheels and tires was 55" wide, so the difference isn't that big, other than the suspension travel.
Scooter, you could probably build a mini-sand rail for a bit cheaper, but like you said...it wont be near as versatile. I do ride a bit on my parents property, so the 4x4 function is nice to have. I did kick around the idea of going with a mini long travel rail, but for 15k or less, you wont find a whole lot that is comparable in function. Yes, you can get the VW powered standard travel dune buggys for a few grand and build off of that, but that wasn't something that I wanted...those are much larger and wont be much good for anything other than the dunes.