Suspension seatposts can become slack and wobbly after heavy use, I dont think much of them.
A better alternative for large riders, if you want to reduce peak loading on your bike, or get a more comfy ride, is a sprung leather saddle from Brooks. They can take heavy loads, and are available in wide fittings.

If you decide on an MTB, make sure you get 36, not 32 spoke wheels.