If this vehicle is going to be a long term investment then I'd be looking for 110 Defender 200 or 300Tdi. You need a long wheel base for the load / kit you want to carry and the 200 and 300Tdi's are far superior to the early Tds and the normally aspirated 2.5. With a Defender you also get coil suspension, disc brakes and power steering which will make your driving experince far more pleasant. If your max budget is 6000 then you are probably looking at a 200Tdi. What you want is a vehicle with proven service history. Re mileage I a have 300Tdi with 150 thousand on the clock and its still sweet. It has however been run on fully synthetic oil from new (reducing engine wear) and serviced like clockwork. Has your average ex utilities vehicle been looked after so well? You also have to wonder about the gearbox on a vehicle with such high mileage.

However if you just want to make the Africa trip and sell the vehicle then I would buy the tidiest LWB ex military Series III I could find. It will cost you a lot less which in turn will mean your carnet will be cheaper. Travelling in a series vehicle is "basic" as you will probably remember. Embrace the fact that you are driving what is now a relic from the past. You should be prepared for mechanical difficulties but in your favour you will find local mechanics are familiar with the vehicle.

In either case get the vehicle fully serviced and prepared before you leave and check it at the beginning and end of each day whilst on expedition. That way you'll catch things before they stop you in your tracks.

I would also read chris scott's book Sahara Overland and Tom Sheperd's Vehicle Dependent Expeditions. I also recommend (dare I say it) that you subscibe to a Land Rover magazine (just dont tell your friends). You'll pick up a lot about the various vehicles and there are also a lot of adverts for vehilces for sale.