The weakest part of any bike are the wheels
MTB wheels have a better strength/weight ratio than larger wheels. In theory larger wheels roll better, in practice, with high quality road tyres, there is little difference between the sizes. Most MTB bikes come with off-road tyres, but these can be switched for slick road tyres when new.
The strength of a wheel is mainly due to its build quality, then its spoke count, then the quality of components. 32 spokes is the modern standard, but 36 spoke wheels will be far stronger. You can get wheels with more spokes (for tandem or expedition touring), but 36 should do.
For practical utility riding, you will want frame eyelets for attatching a luggage rack and fenders. I would suggest that you avoid full suspension bikes, they add to the weight, and sacrifice quality elswhere for your price, but if you want suspension, go for a fork.
Most brands are pretty similar, so pick a bike shop that you trust. Find one with a good wheel-builder and find out if they can check the wheels and "pre-stress" them. This makes factory wheels as strong as custom made ones.