Welcome : )
would there be any behavioral or physical problems with a bunny this old that hasn’t been spayed yet?
Are the owners forthcoming about their reason to rehome her? There may be some behavioral issues (mostly territorial toileting) but I believe most things can be managed. This can happen even with a desexed rabbit coming into a new environment. Usually, the territorial marking eases off.
Intact females may go through false pregnancies and be territorial about their habitat. But they also may be total love-bugs! It really depends on the rabbit and their history.
A spay at her age shouldn’t be an issue with a good rabbit vet. The older they are, the more fat there is around the uterus but an experienced vet should know their way around that.
Have you yet to meet this rabbit? Do you know if the current owners have dogs?
If she isn’t the right fit for you, try some rabbit rescues. Often the rabbits are already desexed and some living in foster care are already socialised with other pets. It would take away some of the worry for you.