Are you set on 40? If not, I'd look at the 19. I've been through all the various Glock calibers and models, and while I still have a couple of 23's, the 19 and the 26 are what I carry.
Shoot it before you upgrade anything. There is no reason to mess with the guide rod and upgrade yourself into some possible failures. Buy an extra OEM guide rod and throw it in your parts bin in case you ever need it, if you want to spend the money on something. As far as the grip plugs, there is a reason the ones for the Gen 4 are not flush and that is to allow the backstraps to be used. Some plugs do not allow the Gen 4 backstraps to be used, so pay attention to that when you are looking at them.
After you shoot it, and if you intend to carry it, start looking at night sights. I'd suggest the Trijicon HD's. If you don't have one, look into a good gun belt and a good holster, then buy a bunch of mags, and a bunch of ammo... then shoot the thing alot.
Yea i had thought about the back strap issue and wondered if it would interfere....ill probably skip on the plug then. Yea i plan on carrying it i used to carry an sd40ve and i had a really ****ty belt....so i went to walmart and found a very nice thick and wide leather belt and WOW what a difference. Much nicer!!!! I wanted a glock 17 the store had but they sold it before i got there and my next choice was a 22 or 23 and they gave me the 23 for the price of the 17. The only reason i wanted the 17 is 9mm is cheaper to buy. But as far as the sights go ive heard that from everyone! If i upgrade em are they hard to install and will it still be sighted in perfect?