Hi James,
I would recommend checking out the tracking capability of the the Alt Az mount you are considering. This is much more demanding for Alt Az mounts as, unlike equatorial mounts where all that is needed is a single axis running at siderial rate, both axes have to run simultaneously at varying speeds under computer control. For low cost mounts you may find this rather than field rotation limits the useful exposure time. I have a vague memory of a low cost Alt Az mount (Celestron I think) which could be run in equatorial mode using a wedge which might be a better solution but this would depend on the mount drive software being able to cope with this configuration.
Cheers
Robin