There are huge design variations available here in the UK - including at least two US designs/makes not mentioned above, Snapper and Wheelhorse.

The general view here would I think be (1) buy cheap - buy twice (I've done that) and (2) Mountfield (3) It may be worth specifically sourcing an OLD Mountfield, which were built like trucks and you can easily put eg Whole new engines on.

Transmissions seem to be usually proprietary and while not economic to replace seemed to be available even for machines about 20 years old(!) last time I checked.

My information is several years out of date.