The really under-rated ones have to be the P3/P30 series.
They're not hugely popular as they need batteries to work (but how hard is it to carry a couple of spare button cells?) and have a lot of plastic trim, but the chassis is metal from lens mount to tripod socket, the meter and shutter are accurate, the viewfinder is bright and displays shutter speed/mode/flash ready with Pentax dedicated flashguns, it has DOF preview and full manual mode. I'd actually recommend them over a K1000 for beginners as they're smaller, lighter, cheaper, have a better light meter and nicer handling thanks to that little finger grip on the front.
The only flaws are that you don't have manual control over film speed (they use DX coding) and early P3/P30s don't have a cable release socket (later ones and the n/t suffix upgrades do). The former isn't much of a problem as you can easily remember to change the settings by a given number of stops from what the meter says.
I've paid as little as 99p for one in full working order before now. £10-£20 will get you one with a lens, although you should check what you're getting as the non-SMC 28-80mm zoom bundled with some of them is terrible. Stick an M or A 50mm f1.7 on the front and it'll handle pretty much anything.