Make sure that the frame has threaded eyelets for a luggage rack (+ fenders), this is essential on any grocery-shopping bike. You dont have to fit a rack but if you do, bolt -on style is by far the most secure.

Entry-level hardtail MTBs in the $250 price band are all pretty good, pick your shop first then see what brands they do. Trek is as good as any.
You may be able to do some upgrades when you buy. Look at changing the knobbly off-road tyres to a 1.5" kevlar banded slick. Armadillos and Schwalbe Marathons are highly recomended for tough city bikes.

Budget for bike, helmet, lights, lock, puncture kit inc spare inner and pump, fenders, [rack] [luggage]
You cant negotiate on price but you can usually get some free LED lights or spare inners, these are retail value to you but wholesale to the shop.

Make sure you get the correct size and check if they tune the wheel build and check the assembly.