i would start with a good used mountain bike that is tenish years old. find one for $100.
disc brakes are great for off road use because they are less sensitive to a bent wheel or serious mud/muck. but at 250 pounds i can skid my front wheel with a 10 year old set of avid v-brakes.
90's early 00 alu. frames were very stiff. great for a road going electric conversion. but too harsh to maintain any value as an off road bike today,
if my local bike shop was a walmart... I would walk.
__________________
|