I'd go for whatever you can get used and in reasonable condition with a rigid frame. I have a 1980s rigid steel 26er that I built out with lower end road bike components that's rad for getting around town/groceries/commuting because it looks busted but works fine.
Someone I know got a Linus Bike Dutchi 3i awhile back and honestly I wasn't really that stoked at first but -- I was wrong, it's pretty amazing. I've even thought about getting one. At first I thought they were just about the aesthetic but it's the real deal. Just a rock solid basic bike for running errands in town. If you don't mind, or prefer, single speed they have also have a barebones fixie/SS model for less $$.
I've got an older trek road bike but I road it today and it seemed kinda sketchy... Didn't love the hunched over the bars feel to brake.
from craigslist, $100 in U.S. slow, unwieldy, unbalanced. 8 gears. High speed is a stroll of leisure, low speed will climb a cliff.
edit: also the brakes are horrendous. I try to avoid going downhill. it's basically like an old ass truck.
any bike with a rack and some panniers will do.
do not use a backpack.