Hi friend, I'm new to the site, but not so much to the game. So far, I've helped 13 people start playing and 13 of them are still playing. By far, the best advice I can give you has been given already. Get a Buzzz. Seriously. It's one of the most beginner friendly discs and you'll use for the rest of your life. It's very straight and very forgiving. You sort of get an "Ok, I can do this!" feeling every time you pull it out of the bag.
As for plastic, just use something that feels good in your hand. I started with a FLX but fell in love with the Z as soon as I threw the first one. You'll want to start in a lower weight, possibly 160-170, your girlfriend might want to go even lighter. The heavier a disc or the "faster" a disc, the more power it takes to make it do what it's supposed to do. If you have friends that play, see if they'll let you toss a few of theirs around. If you don't, make friends at your local course! This sport is best played as a social sport among friends and everyone is looking for others to play with.
I'd recommend a Challenger as a putter, almost everyone has one in their bag because it's incredibly reliable and consistent. I just recently stopped using mine, but I always use it when teaching new players to putt. It's just a confidence booster to be able to fall back on when you get close.
As for a driver, I'd advise you to go one of two routes: play without one for a month or two or get something slow, a 7 speed or less. The first option will guarantee you develop good form relatively quickly, so when you move up to drivers, you're set from the get go. This also forces you to develop your short game, this is the area where tournaments are won. A good clean form will do wonders for your score. A strong short game will be a strong foundation to stand on. The second route will let you get more distance off the tee more quickly, but will hurt your game in the long run. Drivers are harder to control than mids or putters and sometimes newer players will dirty up their form just to throw a driver a little farther. I did this (started with a Boss and a Katana, very fast discs, I thought FAST meant FAR, which is not the case if you can't get it up to speed) and it took 6-9 months to fix the flaws. Do not do this. Slow driver if any driver at all.
I didn't mean for this to become so long, but I assure you, this advice has been good to my friends and I have no desire to see you fail. (avoid understable drivers for a while, either way)
Hope this is worth the internets it's printed on...