Granted, with yer average 2 stroke engine, using yer average recommended break-in prop, you should be fine with no fancy gadgets....
However, with a 4 stroke engine, a tach is more or less a necessity.
DO NOT buy a H9 DVT. I've currently got a Glo-Bee and seems to work for me... The best tach (within reason) would be the TNC sensortach.
The only reason I can think of why you'd need a heat sensor would be if you're experimenting with fuel, plug and prop combos to get the very last scrap of performance out of the engine for racing for example...