Agree with symphony, some additional thoughts:
Would I be able to install another 2 x 140 amp hrs Total 560 amp hrs AGM batteries &
Depends on the available space. All batteries should be close one to the other with shortest connection wiring. Adding a new battery set to an existing one (except nearly new) is not suggested, i would suggest changing all batteries of the house bank.
I am looking at the possibility of going Lithium with a Victron charger/ inverter 12v 3000-watt version which I think will charge the lead-acid engine battery, the lead-acid bow thruster & the Lithium house bank. poss 400amp hrs usable up to 80% can anyone confirm this?
Depends on the lithium cells, i have a 400AH lithium house bank and draw it down to 10% and up to 99%. I leave the 10% for safety as my BMS cuts off the system at 0%, which causes the whole boat to fall into dark. Changing to lithium is not only changing the batteries, but is a change in the whole energy system of your boat. The advertised "drop in batteries" aren't "drop in" because you have a context with your whole charging system.
Would I need to upgrade the alternator to cope with charging the Lithium or will the standard-fit Volvo d130 engine alternator work fine?
Yes, upgrade is neccesary, the original VP 115A will die for sure
! Lithium batteries take every available amount of current and the standard alternator is capable to deliver 115Amps only for short time before overheating and sending the "magic smoke". After upgrading you get far shorter charging time as from the standard one.
buy an AC lithium charger for the lithium batteries?
I kept the quick charger and added a mains cutoff controlled by the lithium BMS. The victron inverter/ charger need a lot of space, cost a fortune and in my opinion make only sense if operated with a (weak) genset.
upgrade to solar install 400amp hr lithium install plus lithium mains charger
Keep in mind, that a lithium project is not a DIY project!! It takes a lot of planning and know how to complete such a project, and in the worst case you will end up destroying your boat by fire.
I'm an electrical engineer and learned it by the hard way too (but luckily without wrecking my boat
).