Lightly used Ls are here and there, too. New or used, this is a good time to shop, especially if you have cash in hand.Congrats on taking the plunge and deciding to get back into riding, it's hard to stay away lol.
In regards to a Dual Sport segment bike it would be hard to long for more than what the 450L will provide, if you need more from the bike then you need a different bike altogether and should be looking at the middle weight ADV segment instead. One of the guys I ride with also has a 450L and he's 6'2 285 lbs and he has over 4,000 miles on his and absolutely rides the piss out of it so I wouldn't sweat your size and weight.
Tires range from $50-125 each depending on brand and model and suspension could be as cheap as a DIY spring swap at $250 all the way up to $1500+ for a full on overhaul/rebuild with fancy coatings and such. I did a complete overhaul with respring and revalve and I was at $750 which is pretty competitive as most shops wanted north of $1K for the same work. I'd set aside $1000-1500 for tires, suspension and a better seat, the rest will come in time as you see what you need and as things wear out.
Call around to multiple dealers in your area, you might get lucky and find some deeply discounted left over 2019's, those savings would cover the upgrades you mentioned.