Personally i would recommend a 200tdi over a td (a td is NOT injected, the fuel's swirled in so it's just a Turbodiesel- and just out of interest these engines are normally referred to as the 19j engine) just because they are a bit (noticeably) more powerful and far more reliable.
You will easily get a reasonable 200 for your funds. If you start looking at the 300 you will start to have to compromise on quality, and there's no real difference between the performance of the 200 and the 300, it's just an emmissions thing. With landys, age doesn't really matter, it's the quality of the important bits, whichis really the chassis an bulkhead (and of course check the obvious, gearboxes, engine etc)
The immediate obvious difference between the td and the 200 is the oil filler cap - in a td it just lifts out and has 3 indents in it and is made of metal and has a domed top with a hose coming out the side, whereas the tdi is a plastic jobby which is flat and unscrews. (and i think is yellow?)
Look on Ebay, and on the Autotrader website, for an idea of what you can get. Check before you buy, and do a search on here for what to look for, there's loads of good info, and if possible take someone along who knows what to look for. As i said the chassis is the really important (non-obvious) thing, a bad one can make the car worthless. CSWs have 6 or 7 seats so you'll be ok there, but the back ones aren't usually great for long journeys. Vans are easily converted to station wagons for only a few hundred quid.
Defenders are slow, chunky machines but they are relatively easy to drive. It is not like driving a normal car, they don't have very good road manners and are not very good in high speed impacts from a passenger point of view (the rigid chassis means the body can't cope with the stopping distances!) but as a first car, relatively cheap to insore, far better street cred for your son than a blinged up saxo and much more fun - and he will meet interesting people who randomly come up to landy drivers and start chatting about them!