Let me start with, I own one of the models you're referring to. While I would love a DIY bolt up kit like the coil spring stuff MG did for the 4th gen models to be available, I doubt I would opt to get one. The 3rd gen just isn't worth it to me, and likely to most others. The down sides include power/mpg options that aren't as good, dual fuel tanks are kinda a PITA, very limited after market support due to it's age and lack of safety features.
I will say that at lower altitude the power was sufficient if not great, but now that I live in Colorado not so much. I can't see adding weight via a 4x4 conversion will help this at all. It just will be less trouble to find a 4th gen to work with long term. I'd like a ecoboost Transit with an MG designed 4x4 on it, but have other priorities for now.