One useful bike to watch out for is the old Cannondale tourer, and archetypal big-guys bike.
A good big bike needs fat tubing for stiffness. Traditional bikes from 1970s/1980s use standard thin tubing in longer length for big sizes. This is not stiff enough and the frame is typically "noodly". The use of fat tubes in smaller frames gives them the reputation for "excessive stiffness"; that is not an issue in xxxl sizes.

The Cannondale tourer is well build, well proven by plenty of big riders and provides a good, practical ride.

The largest production MTB size is 23". You could probably make this fit OK with a suitable seatpost and stem. Put some slick tyres on and it will be OK on the road.
29" MTBs use a touring/hybrid size wheel (700c) which is good for big riders.

For the ultimate Big-Guys bike, see Zinn, his custom bikes are way above your budget but a useful inspiration.
Crank size is critical to correct sizing outside the norm. The largest common size is 175mm which is small for you. This limits your spinning circle, constricts your lanky leg movements and makes you inefficient. A longer crank will make the whole bike fit you better BUT they are hard to find and expensive. See Zinn for more info.