The main corrective factor for the co is the coolant temperature sensor, these do bugger up and usually go mental rich when they do, they cost around £12 and are easy to change, ideally want to change them every 2-3 years anyway.
But as said there are a lot of things it can be, and without knowing your way around a cossie engine, money wise would be best to book it in somewhere so they can have a play.
It may be that it is just a crap setup and not what it appears to be, ie seller lied ?
Either way at that co it needs fixing a lot sooner than later, it's like having the choke fully on!!
tabetha