[Azathoth]

You can get a extended back end for the P90. I can't remember what it's called on the HK websites but it's alike a rubber condom that fits over the back end and allows you use use larger batteries. I have never used it since I went LiPo, but that option is available. You may want to PM illusion as I am sure he has seen or installed those and can tell you what they can fit inside.

When you install the new gears you will need to have the gearbox re-shimmed for the new gearset. If you have metal bushing installed already you are fine.

LiPo batteries need a balance lead and requires it. I suggest you go to a RC shop and buy a charger. They range in price from 30-200 dollars. If you are not a serious RC enthusiast then the 30-50 dollar solution will get the job done. DO NOT buy (my opinion) an airsoft branded lipo charger. They offer poor to no value for the money compared to a charger you can find in a RC shop.

Forgot to mention. DEANS ultra connector or EC3 connectors for a quality LiPo pack. You will not regret going LiPo. It does require some maintenance and care but a quality lipo pack from Reedy/Zippy costs 20-30 dollars compared to the 40-60 I have seen for 9.6v 2/3a batteries that retailers are asking.

My opinion. Don't buy airsoft branded lipo/lifepo. Again, the offer very little value for the money in. I regret buying the firefox/hotpower and various other airsoft branded packs. They under perform to their specifications.