Actually, using propellant ot cool electronics is not so bad an idea, but it's not so practical. Many satellites use a heat pipe system which uses a fluid to carry heat from electronics heat sinks to external radiators. Heat is wierd in space, you'd think it's very cold, but without fluid to convect the heat away the inside of an unpressurized vehicle just stays hot, and the outer skin reflects heat back in.
This is the problem with spacecraft design, it seems so easy but years of experience tell us otherwise. Explorer I, the first US satellite, was supposed to be spin-stabilized. Works on paper, right? Just like a football pass? Nope. Turns out that energy dissipation from whip antennae and fuel slosh complicate the matter, and the vehicle tumbled end-over-end after a short time.
It's little things like this, that aren't obvious, that make it difficult for the backyard amateur to build his own space vehicle.