Yes you can still get vintage...but if it were me I would stick with current versions. Vintage will be more expensive (probably but maybe I'm wrong) and it will DEFINITELY be a lot harder to replace if she really likes it and wants to buy more. (If not impossible to replace).
I'm not a No5 fan so I have no idea as to your other questions, I'm sure someone else can help you out there.
There were some threads in the past. The parfum is crisp and clean, the EDP is a little heavier, the EDT a little inbetween, and the eau premiere is more citrusy. So it depends on taste. The parfum is probably best, but it is very expensive (almost $100 for 10ml). After that, my favorite is the EDT, though it is a little light. The bath oil (currently available only in Europe) is great too (as is the regular shower gel and the soap).
There is still plenty of vintage around on ebay and the like, but it is very difficult to judge it, and it can be expensive, so I would not recommend it for anybody who has no experience in vintage buying. Besides, the current juice is still quite ok.
The nice thing about No. 5 is that you can't go wrong, as all the current concentrations are beautiful. I would stay away from vintage in this case, since it always involves some risk and since the current formulations are all very nice. I think it's fair to say that the scent people typically associate with No. 5 is the one you get in the edt.
Current Top Five:
1. Bois des Iles--Chanel
2. George Sand--Maitre Parfumeur et Gantier
3. L'Ame Soeur--Divine
4. Violet Blonde--Tom Ford
5. Santal Blush--Tom Ford
I have all concentrations in current and vintage. The current formulations have coarsened somewhat, compared to the vintage. If you want to try the vintage, I would recommend going for the cologne. This is rich, strong and has a good lasting power. It is generally more available than vintage parfum, which to the modern sensibility, has plenty of animalic tone to it.
Of the current versions, I believe the EDT is the prettiest, and also the one that keeps closest to the spirit of the original. Like all light formulations, it does not have massive longevity or projection, but that all depends on the wearer's skin. Generally, the drier the skin type, the more it tends to 'eat' perfumes. If your gf is lucky enough to have mediterranean colouring, she will probably get a fair life out of the EDT on her skin. If she is fair, then a light, non-perfumed moisturiser will help extend the life of the perfume on her skin.
Nothing compares to the parfum. It's a masterpiece. I enjoy all the formulations (maybe not so much Eau Premiere), but the parfum rocked my world. It changed how I perceive perfume and completely altered the course of what I now buy and wear.
Even if she ends up not loving it, there's something about that little crystal flacon. The other formulations don't quite have that specialness factor.
Though my favorite is the EDT, it's a bit butch for today's women(I wear it as a man in public and nobody thinks it's a woman's fragrance!). The EDP is hideously dated(which to me means anything from the 80s) and I'm not a fan of it at all. The parfum is intensely musky and a bit murky for my tastes, but still lovely.
All in all, I would recommend the Elixir Sensuel - it's cheap(ish), it has the classic chanel #5 opening, and a positively gorgeous dry sensual iris, amber, and musk drydown that's feminine with a sexy growl. Its format, being a gel-based dab-on bottle, adds a bit to its luxury and allure.
My Top Ten:
1: Guerlain - Habit Rouge
2: Guerlain - Jicky
3: Guerlain - Mouchoir de Monsieur
4: Guerlain - Shalimar
5: Knize - Knize Ten
6: Caron - Yatagan
7: Caron - Pour Un Homme
8: Jean Desprez - Bal a Versailles
9: Yves Saint Laurent - M7
10: Salvador Dali - Dali Pour Homme