Lots of threads regarding the TM G18C out there -
the best I found was on Airsoft Canada where they outlined all the upgrades and discussed it in detail in the later pages. Pro tip: Look into filling in the front frame void with epoxy when you can as it'll improve the longevity of the gun. That front screw post will be the first thing to break on you so long as the hammer doesn't go first.
Gas
On a plastic slide anything cheap will do - Nuprol is easy to get (just the regular green bottles) but the cheapest and most reliable average gas I've used in my pistols has been Smart Gas (from Poland) or Pro Tech Gas from France - both are the same gas but relabelled, and both are about 7-8 Euros per can. Buy a few cans and some BBs and you'll have made a saving over what you can buy here.
Magazines
Cheapest place I've found is
Rainbow8. They have the extended ones cheap too (I'd pick one up as those are absolutely amazing). Shipping takes two weeks and I just received my extra 50rd magazine in the post yesterday.
Holster
Look at Blackhawk. Second handers can be had for £30. I don't think Nuprol deserves people's money on their clothing and accessories but that's just me. Bear in mind that the TM G18C is 1mm wider than the WE and real-steel equivalent so you need to use some sand paper to get a good fit on either.
I just run mine in a WAS generic holster and it works well and is light. If you play woodland more than CQB I'd recommend a fabric holster like this as they take up less room on molle and are marginally lighter.
Torch
Look at the Element X300 clone. Won't perform as good as genuine Surefire ones, but if you're on a budget they're allegedly quite good (remember to buy perspex inserts so it doesn't get shot out in CQB). There are loads of half-decent ones out there though. I'd probably spend your money on something more worth while at first.
Also a word of warning - a G18C with torch
isn't going to fit into a Blackhawk-style holster.
Servicing
There are a billion videos on this, but the gist is: Remove slide and use high-viscosity silicone on the plastic runners in the slide as well as on the metal ones in the frame (one drop on each is enough). One drop where the full auto switch is to keep that running smoothly, and a drop on the nozzle seal (then spread around with your finger before leaving the seal to rest overnight and adsorb that lube or it'll just be blown down the barrel when you next shoot).
Remove the barrel and hop unit (just slide it away from the recoil spring) and then you can use small paper towel squares wrapped around your cleaning rod and isopropyl alcohol/rubbing alcohol/medicinal alcohol to break down any gunk in the barrel and hop unit (keep swabbing until the paper towel comes out clean). Use a small bit of silicone on a square of paper towel and swab the barrel from the exit end downwards, but
do not go as far as the hop rubber (getting silicone on your hop rubber is bad). If you're not sure where your hop rubber is then look down it from the other end and you'll see the nub protruding into the barrel.
Most of the time it's fine though. Only needs a proper clean every couple of months even when regularly playing.
As for magazines, just keep a little gas in them. If they leak then first run some Abbey maintenance gas through them. If that doesn't work then disassemble, soak o-rings/seals in silicone overnight then replace. You may want to invest in a valve key - WE magazine valve keys work fine and can be bought for £5.
Upgrades
The Glock is super simple and I'd encourage you to look at it yourself. That thread I provided has a whole wealth of knowledge and screwing up permanently with GBBs is hard.
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