REVs come with a 151 or 159, the REV XPs come with the 154.
I've had an 04 and now have an 07.
As for years, this is my take:
04-good year, I had no complaints with mine, strong clutching, snappy engine
05-I hear there could of been ring issues, flaking, but good strong sled
06-great year, strong reliable year, hard to find
07-lots of things to check
-first year with the Power Tek engine, very strong engine, but issues with crank shaft bearing clearances in some releases. You need to check the crank shaft run out to make sure its within spec.
-first year they ran without a thermostat, infamous for a cold engine seize. If your crankshaft bearing is within spec, machine the head and drop in a 06 thermostat, install a temp gauge to be able to monitor engine water temp.
- primary clutch issues. Spring tower caps are weak and wear fast, you may need to get an updated version or new one.
- Once you get an 07 tweaked, they run very strong.
In general, the REV chassis is a great handling sled. I personally like my 07 over my 04, but I think I got lucky with mine, no issues to date and 3000 miles on it. Turbo Al checked the crank shaft run out and its like brand new. I put in a thermostat and temp gauge and did some clutch updates. The 07 seems to climb better than the 04, 04 tends to trench more in the powder. Also, to keep belt temps down and hold RPMs when climbing, change the top gear in the chain case from a 21 to a 19 tooth. For all around climbing and boondocking, a 151 works great. Every REV can use better venting to keep the engine compartment cooler. Remove all plastic covers around the clutch, and feel free to drill out extra holes in the shock towers and side panels, use plastic dog screen and plumbers goop from Home Depot to keep the snow out.
Hope you find a decent sled, and hope this helps.
Cheers
