Danielle, the house you have your eye on could be in a nice area or it might not. Do you know the actual address? That would help. Part of Mt. Airy is lovely -- it's more affordable and quieter than Center City, there are more detached houses, and a lot of young professionals and families are moving there. But the section near Germantown is still kind of sketchy. I'm guessing that if it's closer to Manayunk, it would be pretty decent, though it could go either way.
Manayunk used to be really cool and funky but it's gotten too popular for its own good, IMO. Parking is generally a nightmare, there are more big-box stores than boutiques now, and the scene feels very "undergrad" to me. Not that there's necessarily anything wrong with this, but I liked the area better five years ago. It still has a fun vibe though.
There is a train line as well as a bus or two that go through Manayunk, so getting into Center City or to Temple is do-able but driving to campus might actually be easier/quicker if you have a car and a parking space.
One thing about Manayunk you might want to take into consideration, though, is that it takes as long to get into Center City from there as it does from some of the suburban areas, but since it's technically part of Philadelphia, your fiance (and you, when you start working) will be subject to the Philly Wage Tax (a little under 4% for Philly residents whether you work in the city or not). If you work in Philly but don't live there, the rate is about 3.72%. If you neither live nor work in Philly, you don't have to worry about the wage tax (some municipalities do have a local tax, but nothing like the city one). Property taxes tend to be lower in the city than in the 'burbs, though.
Anyway, I think the house you're considering is definitely worth looking at, I'm just not sure I'd buy it sight unseen. Are you coming to Philly anytime soon?