hi johhnny,,6mm ply is too thin to tile onto even if it is in good nick mate,if your fixing ceramics i would replace the 6mm ply with 12mm wpbp ply screwd to the floorboards,adhesive would be a flexible cement based adhesive,dont buy rapid set just ask the supplier for one with a good bucket life giving you time to tile at your own speed,,i would also advise to use the same adhesive for the walls,i would stay away from the tubbed ready mixed adhesives johnny as tempting as they may be for easiness you cant pack out dips and hollows in the walls with them to keep your tiles nice and straight but if you use the bagged powedered adhesive i mentioned then you can with this.
mapei keraquik is a good one and a nice pot life,tile giant sell it and a fair bit cheaper than anything out of topps mate also dont forget to ask for primer from the shop also for your plywood floor mate dont use pva