Honda CR-V fits all those requirements.
1. Bullet Proof / Reliability
My wife is on her second CR-V. It's got about 80,000 miles on it. The only work we've had to have done on it so far not including consumables (oil, battery, tires, wiper blades, etc) was to replace a power window motor. Her previous CR-V was similar, needing only an A/C compressor for the 150K miles or so we drove it. We've never given a second thought whether she'd make it to and from work. And since the CR-V is all-wheel drive, it handles snow covered roads like a mountain goat.
We've transported all sorts of crap in that CR-V -- twin pillowtop mattress and other furniture (actually, come to think of it, we even brought home our king-size Bed In a Box mattress and unassembled base); mulch; tons of Christmas presents, whatever. We just roll one of the back seats forward and we've got instant extra room. My wife reckons Hermione Granger must've cast an Undetectable Extension Charm on it.
It's got a four-banger, so it averages better than 21mpg city / 24 highway. We generally get about 320 miles or so on 13 gallons of gas when driving to Florida from Tennessee.
4. Low maintenance cost / ease of maintenance
Well, it's not the sort of car you can fix with a pair of nylons and duct tape, but the infrequency of needing maintenance easily offsets the cost of having belts replaced or whatever.
Here's a 2003 with < 65,000 miles for $6200. It may not be pretty, but it should be dependable, assuming it's got a clean CarFax report. It's been on the market for over 2 months, so I bet you could easily get them to come down on the price a few hundred. Bonus: it has a cassette player! But you'd have plenty of money left over to install a
lift kit with new wheels, tires, and nerf bars if you want, upgrade the sound system, reupholster / re-paint it... HGTV Property Brothers stuff. Old + cheap + low mileage is just begging to be modded.
Here's a 2009 with leather and heated door mirrors that you might *possibly* be able to haggle down to your $12,000 max. The leather and dual-zone automatic climate controls might spoil you.
Here's a 2011 Special Edition that's been on the market for 3 months that you'd probably be able to haggle down to your $12,000 max. It has leather, but appears to lack the automatic climate controls and heated side mirrors of the 2009 EX-L. Still, it's super sharp.