go on the british gypsum website and look at the site book, tells you everything you need to kno about what product to get. if you want to do the taping and jointing yourself you will need tapered edge boards not straight edge boards. if your going to skim the whole thing, get straight edge boards.

if youve not done this before, do you kno how to make sure the boards are level? if the joists are wet i would let them dry fully before putting new boards on then check the levels.

if the existing ceiling is on the piss, its easy enough to suspend a new MF (plasterboard) ceiling underneath the joists, you will need a frame for that tho, so look at gypframe MF7, MF5 and MF9 on the site book. you dont have to use BG, many companies do the same sort of racking.

if you can get access to a laser leveler, it will save you lots of time!!

i work for a ceiling and drylining company btw