If you were a regular sized guy, then steel would be a fine choice, but you are pretty big, so need a stiffer frame. Al bikes like Canondale are esp good for big guys.
You may have problems getting a good selection of used bikes your size, so if a big steel bike comes along, it will do fine.
Make sure the used bike fits you well.
Ensure that the transmission is not worn out and the rest of the components work OK. Dont worry if it has 7, 8 or 9 gears in the back.
$300 should get you something pretty good, used. Have a look around local bike shops and clubs. You also need to budget for helmet, gloves and shorts.