Then look for a hard tail mtb, with a decent front shock on the used market. Something that retailed for at least 300, like a trek 820, or better still something that cost more like 600 new. What I mean is, something with a real shock, not a non adjustable pogo stick, walmart bike. Put a street tire on it, like comfort bikes run. Or a beach cruiser tire. You might run a knobby back tire, and just have a street front tire, then swap wheels to put a knobby on the front to ride dirt. Too much running knobbies on the street, it just costs you money. 500 miles, done. But it's a dilemma, you just can't do serious trail with street tires. Or worn down knobbies either. Bad tires on the trail means crashing a lot.
Put a 500w rated geared hub motor on the rear, Mac, E bike kit, Bafang etc. Not the tiny geared motor, the big one.
Battery, what ever is left in your budget. 48v 10ah, or bigger. Frame carry type battery, could be in a bag, or a mount on the bottle bolts.
Not sure if you can afford the mid drive on that budget, after buying the bike. But for city riding, I still say hubbies rule, and they seem to be much more reliable than the bbso2.
Chances are, you will end up spending 1500-2000 anyway, everybody does, unless they go with the cheap ass shit. It would work, to buy a $250 direct drive kit on ebay. That might be better, leaving you cash for a better battery. But it won't ride the city as nice, or trail as nice as a geared motor.
If you were aiming at an ideal full on trail bike, then it would be a FS MTB, and the BBso2. And the bike itself would cost 4x as much. $2000 retail new, type bike.