that battery on yours looked really small if i remember correctly. no harm getting a decent heavy duty one for reliable winter cold starts but you do seem to be having a bit of bad run with that car mate......costing you a fair bit.
i wouldnt say that was money down the drain as they have a limited life and chances are if the alternators not been charging it for ages it was past its best.
with GTs its usually the battery or alternator. ive got a spare GT alternator if you want a decent second hand one.....text me.
ive got the senders for the rad fan and stat too.
usually though u thought you got the battery light on the dash to warn you of low charge?
check all your cables too. i remember one cable we discussed on the left side of the bay - cant quite recall what was going on with that..............