Couple of suggestions from a 6'4"/230 pound guy (but I've been as high as 270):
--245 isn't enough weight to worry about. Any half-decent used mountain bike with 36-spoke wheels (even 32s, if you're not going to be crashing potholes and catching air) will hold you. To protect the wheels, run fairly large tires (no smaller than 1.5 road tires, and 1.9 would be better).
--for the kind of riding you've described, suspension is an unnecessary complication. It adds weight and, while it's pretty reliable, it's one more thing to maintain and to break. I'd look for a rigid bike and put on fat tires at a fairly low pressure--that will give you all the suspension you need for just riding around town.
--All my bikes but one are steel, and I prefer steel, but there's no reason you have to have it. If you find a decent, brand-name aluminum bike that fits and works, I'd snap it up. Around here (Reno, Nev.), it wouldn't cost more than $150-$250, and you could cut that by two-thirds if you luck out at a thrift store. I bought my wife a Specialized Hard Rock for a mud/beater bike, in great shape, for $15 at the Salvation Army last fall.