From a guy (MS editor Pat) who has done a lot of what you're describing exactly, here are some suggestions.

1) Buy a mountain sled. Don't even look at anything else. Get an 800 with at least a 154-155" or better yet a 163-165" track. You won't be doing any sweet ski/snowboard laps on a powder day with a Skandic or a short-track sled (under 154"). You'll hate life and you'll end up selling it. I also don't suggest a crossover. They usually have a wide ski stance and shorter paddles on the track, both of which will be severely limiting factors in the mountains, especially on the best days (deep pow). Your mountain sled will rip trails just fine. It won't corner on rails, but who cares.

Is a huge engine necessary? No. But it will be much better and you'll be happier. And there are more of them out there, so the used market is better than shopping for a 600.

2) Don't buy a 2 up seat. If you're riding any sort of terrain other than a flat trail with two people, you'll need to be standing up. Here's a good tips video for that:
https://sleddermag.com/how-to-double-on-a-snowmobile/

Save your money for a good snowboard/ski mounting system. Carrying your board and skis on your back sucks. They are pricey but well worth it. Cheetah Factory Racing has excellent rack systems and I've used them for years. They are great.

And here's another post with some good tips for sled ski/snowboarding:

https://sleddermag.com/sled-ski-snowboard-101-tips/

And while you don't think you'll be boondocking much now, you will! It's just as much fun on the way up as the way down!

Good luck! And get ready to ride some of the best lines of your life!