In my experience (from 15-20 years ago), sales engineers are the ones who really make the sales. The actual sales professionals (account execs, whatever you want to call them) get the glory and the money (and a lot of the stress). So if he would be commission based, that sound great.
First, everything in sales is negotiable, and this will be his first test for selling. I wouldn't really care what the base rate is, but rather what a reasonably-likely income would be for a good performer. And for an SE role, I'd want the floor for income to be $200 with NO CAP on commission.
Many sales companies offer a relatively low base rate and high compensation through commissions. He needs to know how to ask the right questions to negotiate a commission based package. I'm not an expert here, but these are a few things I'd start with:
what is the base pay? (and negotiate all the base benefits including vacation time and how soon to start in 401k)
what is the quota? Is there a cap on commissions?
What is the average take home of all your sales-engineers?
What did the highest paid Sales Engineer take home last year? (ask to see documentation or to speak to one or more SEs.)
What did the last five SEs in this territory make? Why is this territory open?
How long is the sales cycle? How long will the company pay a higher base rate so that he can start up his pipeline?
Think about negotiating a non-recoverable draw against commission for the first year or even two years if a long sales cycle.
One other thing for your husband to consider is that as a sales engineer, he'll no longer get to tell the truth about fixing problems. There are no problems in the sales cycle. Who has to service the accounts after the customers buy? If it's him, he may find a problem between selling and servicing the accounts. When I was a pre-sales engineer, I didn't think of the ramifications of assuming a feature was in working condition. As a consultant after the sale, I often got pissed at the sales engineers for overselling or being dishonest about limitations.
If he has the opportunity to talk to the representative who owns his current account, I'd take him out to lunch and ask him a lot of questions -- technical and businesswise.