An amp is easy.
First I'm curious as to how you intend to get your effects?
If so is are they coming out of the budget or do you already have those in hand?
Assuming pedals, pretty much any 2 x 10, 4 x 10, or even single 15 cab would work. I happen to like Ampeg and Peavey stuff, Peavey holding the edge for value but can't dismiss the brand recognition and tone of Ampeg. All IMO of course.
Used they are both pretty reasonable, even the Ampeg gear, and can definitely stand up to the road.
Don't sweat the amp to much, particularly since you have a sound person and I assume a decent PA. Also with an effects heavy sound don't worry about spending extra money for "tone" nuances or whatever. Just find something at least a few hundred watts or more with a size and weight you can live with and whatever features you are looking for. Some features to consider: a DI to the board/PA, effects loop, compressor, tuner or tuner out. If it was me again I'd give a hard look at Peavey or Ampeg brands (okay, you get that I like these brands but for these exact reasons), but really it doesn't matter too much.
Also if you are using pedals don't overlook a good equivalent size and wattage combo. You may find them easier to travel with, particularly gigging large festivals with a PA. I play a lot of outdoor festivals with my blues band, and my main rig for many years now has been my trusty little Ampeg BA210.
However, if you intend to get your effects with rackmount processor, then maybe you'd want to snag a used poweramp, get a modest cab, and call it a day. I don't do racks so I'll leave it to someone else to make suggestions in that department.
Good luck, sounds like a great opportunity and a lot of fun!