I can't speak to the Powerstation 600 but I know I do own some kind of Spirit Powerstation and its the main system we use for small PA hires, its as you say, really convenient. Sound quality isn't fantastic but its a great starting point. And ours has been ours for about 4-5 years with never a repair, and it was secondhand when bought. Its flightcased though which will obviously make a difference.
You're going to need a modular system as you progress but you can use the Spirit for powering monitor mixes in the future perhaps. Or just buy it second hand and sell it on when you are done.
Small vocal PAs to mid-sized full-band PAs to concert PAs is a massive change at each step up, so much so that much of the gear will become invalid for your requirements.
But you can never have too many decent rack-mounted amps - C-Audio or Crown (same ppl) are a good mid-priced choice - and an unpowered mixer with a rack amp isn't going to be any more effort than lugging around the Powerstation which is a 2-man lift in a flightcase.