First and foremost, especially with knee issues, make sure your fit is correct. Otherwise, you'll contribute to further knee damage.
Second: Learn to spin in a lighter gear rather than mashing a big gear at slow cadence. Your knees will thank you.
As to the bike, at your weight, a good 32 spoke wheel will do you well, and can likely go with a 24 spoke if you keep it well maintained as far as spoke tension and alignment. The frame and type of bike won't really be an issue, other than this: Get the bike you fall in lust with. You'll ride it more.