The extra ammo the battery pack provides isn't removed if you sell the battery pack. So you can buy a battery pack, buy an energy weapon, get ammo bonus, sell battery pack, get battle suit and keep the extra ammo.
Have ammunition automaticilly bought/refilled when using the armoury. There is no advantage to having less then maximum ammo and it reduces the amount of time spend in a menu clicking stuff.
The extra building the humans can buy after their first reinforcements arrive is the defence computer. It look not all that usefull(I might be wrong). Following the description it causes turrets not to focus fire on one target. Which sounds like a bad thing. Short; human extra buildings at tier 2 feels kinda dissapointing. Maybe add something.