Suzuki DRZ400. Long running model for Suzuki in a couple different versions. It started as their off-road trail bike that turned into their street legal dual-sport (I don't think they make a dirt only version anymore). Still in production and has been around long enough the info and parts are out there as needed. Modern enough that it won't look/feel like you're on a dinosaur.
Other bikes for about half your budget:
Honda XR400R (old school air cooled 4 stroke, reliable trail tractor, no radiators to smash)
Honda XR600R (old school air cooled 4 stroke, reliable trail tractor, no radiators to smash)
Kawasaki KDX200 or KDX220 (probably the best 2-stroke beginner/intermediate trail cruiser)
Downside is all those were last made in mid-2000's, so even a clean one might need a good once over. Upside is they are were quite popular and well regarded trail bikes so there's enough info and parts out there as needed.
The newer off-road models now (KTM, Yamaha WR, Honda CRF-X) are pretty sweet. They're all based on the motocross bikes, this is good and bad. You're getting modern everything on it, so no excuses if you want to look and go fast, they will run circles around those other bikes. They'll probably never end up as reliable as those old trail bike because of it though.
PS - Avenza on the smartphone works pretty well, you can download the pdf maps right off the MI DNR website to run in GPS-only mode.