Battery and upgrades for TM Sig 552?
As is probably evident by the question and area I am posting in, I am very new to Airsoft. A guy in my town is selling a Tokyo Marui Sig 552 and I am seriously considering buying it. But it does not come with a battery or charger, and from what I've gathered, it is also a good idea to upgrade these guns to get more FPS out of them.
So more specifically, my question is, what exactly is the best battery to buy for one of these, taking into consideration I want to upgrade it a little? I am aware that these will only take smaller batteries, 8.4V or something like that. I am not planning on modifying the battery compartment or anything, so again, what would be the best one I can buy that will fit? And while I'm worrying about that, what would be a good charger to go with it? I'm trying not to spend much here, but at the same time, I don't want crap that won't power it well enough or die too quickly. Hence this question.
And the other part of this is upgrades. I want to get some more FPS out of this gun (Maybe around 350-450 if possible? I'm not too sure if that's realistic...), but being completely new to airsoft and all things involving maintenance and upgrades for guns, I am not exactly sure what I need in order to achieve this and exactly how I would go about it.
The sig 552 seems to be somewhat popular (relative to other guns I've looked at, but again, I am very new here), so if there's anyone out there who has any experience with this particular gun and can tell me whatever I need to know regarding the battery/charger and upgrade parts and how to install them, I would greatly appreciate it.