I would recommend giving it a full service all round and do the cam belt too.
Fuel sedimentor bowl under the boot floor is worth cleaning out as well as checking to see if your wastegate is stuck open on your turbo, if it is then it will never spool up as it is relies on exhaust pressure and the wastegate vents off exhaust gas pressure to reduce boost pressure.
Just looking at your truck, it looks fairly well kitted out, what size tyres are you running? have you checked to see if the previous owners have changed out the original 1.22 transfer case for a 1.4 Defender unit? Wouldn't explain the lack of get up and go though as it would reduce the gearing it would be quicker off the mark.
Certainly get the cam belt done and set the timing correctly.