Heres a little something to be getting on with......Firstly they are nothing like a car to drive or use, they are a 4x4 workhorse that happens to be ok on the road! So your first drive may be a shock. (not that their horrendous but some people assume they are like a car but a bit higher up!)
I wouldn't bother with anything pre-"Tdi" engine (1990 'H' reg onwards, badged Defender, "200 Tdi" from '90 to '93, "300 Tdi" from '94-'98, both same capacity and power, "300" is more refined and also the Defender got rear brake discs and revised gearbox at same time as change to 300 Tdi) as the Tdi is a vast improvement in performance and economy over previous engines and is well within your budget.
Go on condition more than mileage, example an ex-quarry/farm vehicles may have low mileage but 90% are rust buckets. I'd aim for a 94-96 300 Tdi Hard Top with under 100k for your budget and should have change to do some modifications. Things to look out for are rust to chassis (mainly rear half) and to bulkhead (normally top rail below windscreen and to the door pillars). Engine wise the Tdi in "300" form needs to have had the cambelt modification work carried out (if no proof budget £300-£400 to have it done), "200" doesn't have this problem. Other than that occasionally head gasket can go, turbo can wear and consume oil, lift pump failure quite common. Generally though the engine is very robust and will do 250k+ without a rebuild if maintained. Gearbox's do have some issues with mainshaft so make sure it doesn't clonk to much (gearbox quite agricultural so is slow change and can be jerky if your not used to it), and 2nd gear synchro on high mileage. Door frames are steel so look around bottom of doors for rot, depending if they can be repaired or not they are expensive to replace. Swivel's (bits at either end of front axle) can leak fluid if seals are worn, not expensive in terms of a new seal but time consuming to do. Interior trim not the best to start with but everything is available, and if your doing long distance trips you'll probably want to upgrade the front seats anyway, also soundproofing can be fitted. Key things really are checking for corrosion and that its been looked after.
Try and get a Landy Specialist to take a look at the vehicle - loads about - go in eyes open and you will never look back - probably the best 4 wheels you will every buy.