to work out amps from lights, divide the wattage of the lights by 12v, so a 100w driving light would be 100/12=8.333a. another way is to get a multimeter and put it on the amperage setting and run it as part of the circuit to work out the particular amp draw. keep in mind these are usually limited to 10a, but you can get clamp on amp meters, or even a needle gauge type that you could fit in the circuit of what you want to test. if you look at the battery it should have a sticker stating cca and rc (cold cranking amps and reserve capacity) the rc is how long in minutes the battery will last with a constant 25a draw on it.
i have a hid setup on my motorbike and wanted to fit heated grips. i thought of tapping into the headlight circuit so they would turn off when the ignition was turned off, so they couldn't flatten the battery. so i did a cold start test for the hid using a multimeter on amps and to my surprise it drew less than 1a, even on a cold start and that is a 35w hid setup.... so i guess they do use a lot less than halogens, considering the headlight circuit has a 10a fuse in it...
if you want to reduce power usage on the lights, get hid's for the spotties, they use a lot less power than halogens of the same wattage, as well as putting out a lot more light. light bars (depending on the size) don't use a lot, to give you an idea of mine, when i put the high beams on (22" light bar, 2x hid spotties and 2x halogen spotties) the volt meter i have in the cab only drops .2 - .3 of a volt, that's with the engine at idle. then once the hid's are warmed up it goes back up to where it was before turning them on, even though they are still running.
the more batteries you add, the longer it will take to charge them all. what model have you got? you could price up getting yours reconditioned or see if they have one on the shelf they could mod and do an exchange on yours. to reduce more power usage, you can change globes to led also, but if you do indicators you will need an electronic flasher unit to stop them blinking fast as if a globe is blown...
as for working out how many amps you consume, above is the worst case scenario. realistically you won't have the heater going flat out all the time, the rear demister going all the time, phone chargers don't use a lot of power, maybe a couple of amps...
in all honesty you will be fine. if you don't have a big stereo cranked up with subs, etc going the alternator should keep up with no problems. highway driving, the rpm you are doing is close to the peak output for the alternator anyway, so you will be getting plenty of power out of it...