So... Not knowing very much. My opinion (worth what you pay for), is that you should stay completely independent financially. That could be the living arrangement or the Pre-nup agreement. It sounds like that is where the issue is. What I mean is that you both own homes and are fine on your own. Why mix assets and incomes?? If you want to live together (as it sounds like he would move in with you). Then split utilities and food, etc in half. You pay your mortgage and he pays his. If you are really moving in together (he is moving out of his place) than I would have him rent his house out and and pay you a reasonable rent to live with you. If it is done fair and reasonable then there is no reason to mix anything. If you split, you just both go your own way.
The shop is the big question. What does he use now? Could he rent something? Why does he NEED a shop? Is there a small shop that could meet his immediate needs and you would be willing to build (pay for and have be part of your property) that he could use (added into his rent). Again, then any day or years down the road it is a clean split. If designed right it could be added onto later (I.E. Extend the shop to 2 or 3 times it's size at a later date when you guys decide to share everything or you have the money to build it and he can pay more rent for it...