My DH & I have bought and sold a business, but very different industry.

The fact that your DH works there means that he should have a fairly good idea of what $ the business is taking. If his not sure, it would be an idea to start taking note of no. of customers a day and average sale. It would be a good sense check against his books, when you get that far in the negotiations. It would be a good idea to do some research on how mechanic businesses are valued as well, is there an industry standard? You obviously want to make sure you don't pay too much for it upfront. Also, assuming you'll need to loan the money to buy the business, find out from your bank what % they lend for this type of business, it can often by quite low, particuarly in this climate. Perhaps vendor finance (or part of) is an option? The only other thing I would say is understanding any weaknesses in skill level, as you say your DH is just completing his apprenticeship.

In regards to customers leaving, I am sure there are things you can do to transition, i.e the current owner agrees to stay on as an employee for a period of time (or perhaps that may not be the best thing, depending on the person) and that he signs an agreement not to start another business for x years, even though his retiring, it just covers you.

The other thing to think about on a personal level is the hours involved and the stress side of things. I think having your own business can be great as long as things are going as 'planned'. Don't leave yourselves financially tight to start with, is the only other thing I can say.

Good-luck