PID: I built my own dual SV unit one with a more programmable time/temperature profile PID and one with a straightforward hold X degrees. I find that for SV I rarely use the more complicated feature set. I went with a dual profile so that I could do meat and veg simultaneously (say one at 55C and one at 80C). Pump: Another thing I would consider is circulation. If your using a rice-cooker style system with water you're likely fine with convection however if the heat source is localised and your water-bath large or you intend to use oil or butter you probably want to include a circulating pump, I found some decent heat tolerant pumps at www.lightobject.com. Thermocouple: The last comment I would make is that you want to consider making your thermocouple (PT100) removable/replaceable as it's easy to accidentally tug the cable and stress/break the wiring. Hope this helps.
