NA 1000 are the most reliable. 800 have a much weaker design, with no center crank journal/bearing and are way lower output as compared to 900/1000 (obviously the 800 is the smallest displacement, but even in terms of HP/cc they are behind). 900 had issues, mostly due to dust/particle ingestion but just in general they seemed to have top end issues. 1000 were a big jump in terms of HP/cc, suspension, etc. The 900 also had the most trans bearing issues, AFAIK, but that did carry over to 1000. Honestly, even the XPTs (which are NOT 1000s, they are 925cc and nothing in Polaris branding, literature, or anything on the vehicle itself say 1000... they are NOT 1000 Turbos, lol) are pretty damn reliable and are another big jump for the platform. Obviously budget is always going to dictate things, but if I could find a good deal on a '17 XPT or '18 XPT Fox Edition that might be a sweet spot.
But assuming a XPT is out of budget I'd still shoot for a 1000. At this point an early 1000 should be pretty cheap, more than a 800 or 900 sure but not a *lot* more. I have a buddy with over 13k miles on his 1000. And I know a lot of "high mileage" 1000s out there. My buddy still owns the 2015 XP 1000 I had and it's been basically flawless, now at 3500-4k miles. The only actual failure was the primary clutch that cracked in a high-water crossing (I had an aftermarket cover on it that had a vent with a mesh cover... generally great, but bad for water crossings). I assume hot clutch + cold mountain stream water caused it.
-TJ
