If you go with the Gen 4, then make sure you do some research online. Glock has issued a new recoil spring free of charge for the Gen 4 guns. There are various markings that will tell you what spring is in the gun you bought, but I would suggest having the seller tell you what version is in the gun you're getting, and if its not the newest version, then ask the seller to replace it first. Its free from Glock, but they're not supposed to ship gun parts to individual owners outside the US. They sent me a new recoil spring and it came across the border fine, but some have gotten seized at the border. Your seller should be able to get the replacement part with less questions.
You can never go wrong with a Glock. Mine is a Glock 17 gen 4, never had any issues with it, and there is definitely a reduced recoil from a gen 3. 9mm and .40 don't exactly have punishing recoil to begin with though, so if cost is an issue, then the Gen 3 will be a great deal. I would stick to the RTF though, RTF2 was very rough from what I hear. Enjoy your new Glock, and make sure you get lots of shooting in, a gun is only as good as the training its owner does.
The only upgrade I recommend immediately is the sights. I personally run Trijicon night sights. There are lots of good options out there, but Glock factory sights are plastic and not adequate.