Hi Guy,
I agree that buying a machine is personal. Learn the price range of the machine and arrange a gift card type of gift for the machine.
Your wife has a Viking Platinum. The step up would be a viking saphire or the embroidery machine would be a topaz. I stepped up one level at a time as I could afford it along that same path.
Yes, stick with the same brand she loves. She will be able to use ALL the machine feet and accessories from the platinum on either the saphire or topaz. That will save a lot of money and frustration as you already pointed out, she will know her way around the new machine more easily.
Get the prices on the trade-in or full price and figure out a great way to gift her the new machine, but let her go in to the husqvarna viking store and pick it out.
You are the best!!