Duraboard shed insulation

We have a Duraboard shed (4.5x3m) for the past 9 years. It's a timber frame with a vapour barrier. The exterior is then finished with a cement fibre board. It's fitted onto a concrete base. Last year, I put rockwool on the interior and this year, I'm hoping to finish it with plasterboard/insulated plasterboard. The whole idea is for us to move the freezer from the utility room to the shed or to place the heat pump dryer there if we manage to keep it warm enough. Also, I'll add a bench to lift some weights there during cold weather. Any recommendations on the type of plasterboard to use that won't break the budget! The normal plasterboard you get in B&Q seems affordable but, I realize it's not insulated.

Any suggestions?