In order to replace it with a comparable reg, you need to specify the 1st stage. Another issue is if it's the older G250 with the metal air barrel or the newer G250hp which is all plastic. If it's the older style and in good shape, I'd consider just getting it rebuilt as those regs are very highly thought of and sometimes have a better resale value than the newer ones. (I think!) The metal air barrel helps with cold water performance and also helps with dry mouth.
If it's an old MK10 with the older style G250 2nd stage there are lots of divers who would claim that no matter what you spend, you'll have a hard time finding a reg at any price that performs noticably better. Certainly if you replaced it, she'd have no trouble selling it. Also, I'm wondering if SP is discontinuing the G250 as I don't see it on their UK website anymore, but it's on the US site.