ok, i'm gonna try and change your mind last minute.
from all my research from the glocks, i have found out that the glock23f is newer and has less flaws on it. there is a common problem many people have with the glock 18c. For many of the glock18's, people say that the slide will cycle and almost return to it's normal position, stopping short therefore not cocking the "hammer" and the slide must be pushed forward for a click sound. this only happens on semi, full auto does not have this problem.
there are also a couple more problems with the glock18 out there that are pretty common so watch out for that.
if your glock18 or wtv comes with a plastic loading ramp, you can get a metal one for a couple bucks i think. shouldn't be that expensive.
i think that the glock 23f feels better compared to the glock18 because of it's compactivity and how there are nice grooves for your fingers. g23f is also more compact so everything is.....more compact. lol. so it gives a heavier feel not liek the g18 where everything is spread out more causing it to feel lighter (which it is) and more like a "toy" as mentioned above.
performance wise, the glock 18c SHOULD have better range, accuracy, and higher fps since it has a longer barrel. but just by a bit. however, you must remember that the slide is also longer and supposingly heavier which will take more gas and also put more pressure on its internal parts. but that's just my theory.
now the thing about upgrades and parts. he g17 and g18 are the same size just the diffference between full auto and no full auto capabilities. same witht he g19 and g23f, so all the parts should fit.
i myself was in your position about 2 years ago. i liked the glock19 but i wanted full auto capabilities and i wanted to get the kwa g18, but i wanted a little more compactblity so after a LOT of research, i ended up with my kwa glock23f which i've had for almost an year. i love my glock, i think it was worth every penny of the 135 bucks that i got it for. there are a few minor problems that i was able to fix like double feeding, but it was no biggy. fixed it right back up.
even thought my g23f has a rather short barrel, the accuracy and power (when it was stock w/ no upgrades) was awsome. it would out shoot many stock aegs. including my friend's ca m15a4.
after a while, i put some upgrades on it: 6.04 tightbore barrel, enhanced recoil spring, firefly rocket valve(an issue for automatic glocks, the cheap, plastic, stock, valve in the chamer breaks after a while on full auto, mine broke, but i immediatly replaced it with the firefly rocket valve which is metal) and hurricane hi flow valve.
my glock now shoots at 350 to 375 fps, with a LOT of recoil, sound, and extreme accuracy (for a pistol)
i'm not trying to discourage you from buying the g18, i'm just giving you my opinion and conclusion after all my research. the glock18 is a really nice gun and are very succesful on the field, but i think that g23f works better... i have friends with g18's and they are nice and shoot well, not many problems. it's still a nice decision if you choose it over the g23. if you still like the g18 over the g23f, then go for it. but just remember wat i told you in this fricking long post i'm giving you here. it may sound like i have nothing to do and that i'm full of my glock but i'm writing all this because it would have been nice if some told me all this saving me lots of time when i was doing my research.
btw, you can get "the goul" on AR to polish your outer-barrel for you if you want. he does a really nice job.
have fun choosing.