Another consideration is how much do you really intend on riding? Do you live where you can easily ride? Are you just making a few trips a year west? Newer, better condition, lower miles is better, new is best. I buy new ones every couple of years. I ride 1000-1500 miles a season, all mtn riding, Cat sleds last me 2 years, polaris' last me one year. I have seen guys go 3000+miles reliably on Doo's and Cat's. By that time though you are overdue for lots of maintenance. Clutches need rebuilding, bearings need replacing, hyfax, shock rebuilds, various suspension bushings, motor mounts and time for that motor refresh. You can sink a LOT of money into an old sled just to keep it running reliably. If you do not keep up and you ride a lot and ride pretty hard, plan on a fix it budget to go with the bargain priced sled, just like a used car or truck.
I would pass on a 600 machine for anything above 5000ft elevation. I watched my 140 lb buddy cuss and struggle on his sons Doo 600 146 trying to keep up with 800s ridden by guys packing another 80 lbs of body mass. This was on a hard pack spring day, just find an 800...
I think if you are serious about getting back in you should budget $6000 and find a '16 machine in good shape. Any brand will likely do, the 16 800 Axys is a super fun machine, the T3 Doo's were a big improvement that year and the the '16 Cats have a new front end and 3" track on some. Any of those sleds will get you excited about being back in the sport. I think it is VERY much worth the upgrade.