if you want reliable, then new is the way to go 100%
and if your not into fixing things, NEW with extended warranty is even better idea here!
ALL snowmobiles need yearly maintenance, if you DON:"T do the required pm's ever yr, your lowering the reliability of the machine
so, that means if your NOT doing them, your PAYING someone to do them
NOT doing them is NOT a great idea if your thinking long term ownership or reliability!
that said, about ALL sleds today are about as reliable as any, there really are no BAD one's over each other
SO this ends up with you picking a sled that fits your needs more than trying to find a special make model claiming to be the most reliable!
they all have the same wear and tear on most parts(4 strokes have longer time frames between top ends , its not a HUGE difference other wise on one working more reliably over another, both types well kept will be the same IMO)
every maker makes nice higher end two up sleds
DOO and Polaris, offer however offer models that have wider tracks, that maybe will work better for your plans of a lot of off trail for two riders
you may NOT think you need a larger motor, but most all sleds of this type only come with larger motors
so your most likely going to get a larger motor
a larger motor will just do everything easier, less straining on motor and MIGHT actually increase reliability due to your not running things harder to do the same things
a smaller motor will be using more RPM's most likely to pull two adults on as sled
so, if again reliability is your top goal, larger motor's are NOT a bad idea!
the Polaris Titan or a Ski-Doo Expedition Xtreme might get my vote here for you, larger sled handle two up easier, wider track, prevent you from getting stuck off trail with 2 up on it!