They don't care if you don't complete the process and have to start it up again later. If you're paying the fees and have all the necessary documentation, that's all they care about. Like Sara said, you can drag out the process as long as you want to. And as she said, you activate the green card by presenting the big brown envelope. That means he can still travel as a tourist until you decide to give them this envelope. (Once you receive the envelope, you have 6 months to activate). If you go through this entire process and don't activate, then decide to do it again 5 years from now, that's your money. You can explain to them why you ended up not moving the first time. Like Sara said, the US Embassy is a lot different then dealing with UK immigration. The embassy people called me on my mobile and asked me questions if they needed clarification. When we were at the embassy for the final 'interview' there was a guy who to me, sounded like he was trying to get in illegally (yes, we could hear other people's conversations). They were giving him another chance to get his paperwork correct. This after they had already given him 2 chances. Out of the other couples that were there for the same reason we were, we were the only ones with all the correct paperwork. Everyone else had to bring stuff back.
If I were you, I'd start the I-130 when you think you're ready. Once that's approved, your husband can try looking for work before you do the medical and start the next step. Depending on what he does, you might want to consider having his CV 'Americanized' by a professional.