I've owned both Charmate, and Weber charcoal BBQs; and the latter is (or at least was when I bought this one 15 years ago) much better build quality. But if you can keep them in a garage, ideally out of any damp - or worst case under a cover and give them air time they will both last just fine. There's absolutely nothing wrong with Charmate's products, and some stuff can be improved with BBQ gasket seals etc if necessary; so bang for buck Charmate these days is absolutely the way to go.
Only reason I gave my Charmate to my little brother is because I went ham & bought an Oklahoma Joe Bronco which is a whole different class altogether and really put the Weber stuff into a brand overpriced category for me. I do really like their Q series of gas grills though.
One of the big things for internal rust is to make sure the ashes etc are removed once they are cold and not left in the BBQ for a long time - as they attract moisture and cause the fire grates to rust etc. And counter to most kiwi's attitudes to their BBQs, take the cooking grates out to clean them & let them dry after every cook too :)
If you haven't already - get a chimney fire starter as they just make the whole thing so much easier and faster to get going!