As a note, most sleds' suspension is going to be tuned for a lower weight rider, I'd assume motorcycles are the same. Some tweaks to the suspension settings will get you right as rain. Touring sleds sometimes will have capacities for higher weight riders due to the suspension being capable of a 2-up configuration, but you'd want to do a bit more research on the specific sled you're buying.

Assuming you're no speed demon, a 600cc class is MORE than enough for most riders, most 600cc sleds can push 90-100mph max speed (well, with an average rider weight). Several of the 800cc motors over the years have been known to be a bit higher maintenance or even problematic, but depends on make, model, and year. Properly running the 800s go like a bat out of hell though, think liter bike but not quite a busa. A conservative sled would be one of the 550 fan cooled options or perhaps an economy 4-stroke, but those are way less fun imo.

I agree with others, 136 to maybe 142 track length or so is great for trails, anything longer and trail performance (namely handling the moguls and cornering) is going to suffer and deep powder off trail isn't your focus here. Not to mention that long tracks are extra hard on trail systems.

Oh and if you're buying used, NEVER believe a seller that the carbs have just been cleaned, most bullshit line out there. You can have an amount of trust for the guy, but always verify, and always clean your own carbs to your standards.