I don't recommend Jiffy because it dries out quickly. And I've found that I can do just as good with yogurt cups (I don't have to transplant as often), miracle gro potting soil (instead of special starting soils), and no
greenhouse what so ever (they usually get to humid).
peppers don't like to sprout until it's in the 80's. I sprout mine by putting a regular heating pad underneath them. It worked like a charm. Much cheaper than 40.00 bucks. A south facing window is always good. Tomatoes are very easy to go so I definitely wouldn't do any thing special for them. As far as onions it depends on what kind you are talking about. Green onions i direct sow, Yellow, White, or Red onions are usually grown from slips rather than
seed although I've heard it can be done as long as you are ready to start them early and harvest them all at one time.