Putting an aim point on your G36 is accessorizing, not upgrading.
There is no point in putting on a K kit unless you feel you want the look of a K. You get some minor potential for performance improvement with a K kit, but not worth buying the kit just for that.
007's info is out of date. The 600mAh refers to back when 600mAh NiCd cells were the best cells mini-type batteries could be built from. Fortunately, that's no longer the case. What you want to be looking at are 8.4v mini batteries built from NiMH rated 1200mAh or better. Pick up two and you're basically set, even if you're a hoser (haha, I funny). There is no point in going with a 9.6v battery unless you significantly upgrade your gun. A 8.4v mini 1400mAh will pull up to MS110-equivilent springs just fine.
If you're really worried about using mini batteries, there are two work-arounds I'd recommend. One is the G&P KV stock. It replaces your existing rear stock with one that has a length adjustment feature and the ability to house a large battery. It can be folded like a normal G36 stock, but you need a specially arranged battery in order to do that.
Alternative, there is a special handguard by Classic Army and First Factory (??) that replace your stock handguard. It also fits a large battery.
The only upgrades a stock TM G36C needs are inside the gun, and they are optional upgrades, not essential ones. If you're really new to the sport and only have had the time for a little while, it's recommended that you play half a dozen games or so to get a feel of what sort of performance improvement you'd want. As opposed to diving blindly into an upgrade job.