I've had an ipage site for several years and have no complaints. Email has been reliable and I've had no service interruptions. Be aware that the introductory cheap price doesn't stick around....at the first renewal you are full rates. As a result, I suggest buying into the longest initial term available.
Have you used the database or connected to quickbooks?
I've deployed mysql and joomla to the site (and gallery....and a few other packages), but not connected external systems to it. I see no reason why it'd be a problem though.
Quickbooks (in my not-so-humble-opinion) is a PITA. I've given up attempting to build any kind of interface into it.