Depends on your budget mainly, but few things below:
- metal rather than plastic body (as generally more robust, rust not an issue as can be stripped back and painted)
- Widest cut and most powerful engine you can afford (as means it will cut more at a time, and more power means it won't clog / slow down)
- Pick a branded lawnmower (so that you can get spares easily), e.g. Mountfield or McCulloch are decent lower-mid range brands, with something like Hayter as a premium brand.
- Either a Honda engine (a lot more expensive but more reliable), or Briggs and Stratton engine (cheaper, less reliable, but very easy to fix) - anything else generally isn't worth considering.