Look for one of the 2008-2012 era Globe bikes (an affordable hipster marque owned by Specialized, but discontinued around 2014 or so). There were three or four models suitable for commuting, errands and hauling groceries, etc. They have long wheelbases and chain stays so there's no risk of heel strike with panniers. I see 'em on craigslist in my area for $75-$200 in very good condition. You can find sample photos on Bicycle Blue Book and Google image search.

I have a 2008 or '09 Globe Carmel. Great inexpensive errand bike and for cruising around 12 mph. Heavy duty, massive aluminum frame and it has a suspension fork which is comfortable but adds about 5 lbs. As I ride it, with rear rack, heavier tires than the originals, water bottle and basic tire patch/tool kit it weighs 35 lbs. Much of the weight is in the heavy spring suspension fork and massive seat post (necessary when nearly fully extended) and heavily padded springy saddle. But it's comfortable so I keep it that way. Occasionally I ride 20 miles or so at a time, including some gravel trails, and it's a pleasure on rough roads. I did lower the stem and replaced the original riser handlebar with a flat bar, which was more efficient yet still comfortable.

Globe had similar compact frame bikes with sloping top tubes and rigid forks: the Live, which was fitted with a front rack; the Haul, with integral rear rack; Work, with no racks but I think it had fenders.

If I'm recalling correctly they didn't offer a model with both integral heavy duty front and rear racks, which seemed odd. The rigid forks might be able to handle a heavily loaded front rack with better stability than the spring suspension fork. I usually don't carry much weight on the front of the Carmel, no more than 10 lbs.

I'd definitely recommend the compact frame models with sloping top tubes for easy mounting/dismounting with a loaded bike. I can hoist a leg across the top tube even with the rear rack is loaded up so high I can't kick a leg back across the rear of the bike. Much easier than with a taller horizontal top tube. Not quite as easy as a step-through or mixte, but close enough.

And there are tons of similarly made hybrids from Giant, Trek, many others. Look for a longer than usual wheelbase -- these are sometimes described as comfort hybrids. Besides handling a heavy duty rear rack this also allows plenty of room for panniers and baskets without heel strike. If you're hauling enough stuff you'll soon want panniers to keep the weight low. Heavy loads on the rear rack can be exhausting to balance while climbing hills in hot weather. The same 30-50 lbs evenly distributed between panniers and the rack will feel much more stable on public streets shared with even light traffic.