Mate,
to IIS!
There is some good advice here. But, you want to surprise her? DON'T BUY IT FOR HER!
Let her make that decision.
As mentioned, the best scope is the one that best suits & gets used. It really is a personal decision, just as much as the fellow she's dating is.
That you are going to buy it for her is surprise enough!!! Especially when you don't say anything & take her to Bintel here in Sydney one Saturday morning, and just say to her "Baby, go wild! (but to a $2500 limit!!!)"
Remember that she is the one that's gonna need to lug it in and out, so if it is too big she will struggle with it.
The surprise won't be lost by taking her to the store. But I'm sure it will be better in the long run to let her chose.
Bintel has a banner in the right margin of this site. Their advice is second to none, and they will look after you two very nicely.
Ps, you can afterwards take her to a nice lunch at the Fish Markets that's across the park from Bintel before heading home.