I dont know how you could possibly surprise her and get exactly the machine she would love to have.
My inclination is to go with the gift voucher and take her out, make a day of it, she can try out a bunch in the price range
alotted. There are good choices out there for almost every budget.
I favor Bernina, and then Baby lock. I like certain Vikings, as well as Pfaffs.
She is very lucky to have a thoughtful husband.