Wait do you have to buy a relay for those PIDs as well? Does that mean they're pretty messy to set up?
TBH I'm fine with +/-1C variation, but just over 2C with my current setup is a bit much.
Depends on that slow cooker - if it's a few hundred watts then you can wire it straight to the PID - dependant on PID (as rurwin's post). Otherwise you will need a relay (solid state best: more reliable, no clicky clicky). But this can go in the PID case (which is another purchase!), it's not messy, just another component.
As for PIDs, n2006p are fairly well regarded for the price for all sorts of DIY stuff - SV, reflow ovens, kilns, brewing etc. Got mine for 17 quid incl (Hong Kong job). Has a 3A relay, accuracy is 0.2FS%. Description of a typical set up here (he does go on a bit, but it does show clearly how to control high wattage device.)