Time is money. In my opinion, it's well worth the $100 or so difference just to use Enterprise. If they can't afford $500/seat for Enterprise, they obviously can't afford to start a business.
If they have 2 users, they should use Enterprise, because they will need some sort of synchronization. Maximizer Inc will sometimes object, but then ask them how they can synchronize the database...silence.
Besides, if they're successful, they'll probably have a 3rd or 4th user soon anyway, and will save a lot of headaches by starting with Enterprise right away.
Now if you are talking about SQL-> Enterprise Pervasive, I have no experience, but I would expect it to work if they just backup the old database, then restore and upgrade in the new system.