Which Sealants and adhesives?

Recently bought a '92 frankia 730 but some how missed what now appears to be quite extensive damp in the back wall around the window, the whole panel flexes and needs rebuilding

that said I'm not frightened of a bit of hard work and it must be repaired, I've already sourced wall board @ £45/8'x4' sheet (expensive or not?) but not sure of the best sealants/adhesives? for each stage, looking at the screwfix catalogue but that hasn't helped...

mastic for outer joints? sikaflex to bond internal bits together? oh I don't know

so suggestions of what and where to get it would be greatly appreciated