You really need to have a look at both vehicles and compare them well. A 1983 110 and a 1986 110 could have quite a few substantial differences and I am quite prepared to think that the 1986 will be in better condition (although this is not always necessarily the case, especially in warmer / dry climates). 90's and 110's were renamed the Defender 90 or Defender 110 in 1990 and it wasn't until around 1992-94 when Land Rover started to skimp on build quality. Many of the vehicles produced within that time frame suffered from rot particularly around the bulkhead and the rear end of the chassis especially - perhaps Land Rover were using a poor quality steel? Later models (1993 onwards) also have a chassis manufactured from thinner steel and as such, it will rot away quicker. In most cases, the structural integrity of a Land Rover really comes down to it's previous owners, the first one in particular. If it has been regularly waxoiled or received some form of rust prevention inside the chassis / bulkhead and out, then it should be alright. If one of the vehicles for sale is ex-military, it will more than likely have received the relevant attention but it might cost you considerably more, so do find out.
As for the engine, you really will begin to hate driving it if it's only a 2.5 N/A, especially in a 110. I doubt it will be a 2 1/4 diesel - they are very rare to come-by, even in the earlier years and for good reason. You do need to find out whether or not it is a 2.5 N/A (12J) or a 2.5 TD (19J). I expect the '83 model will be a 12J and the 86 model could well be a 12J or a 19J. The reason being, both engines are particularly under powered, but the 12J will prove to be a much more reliable engine and it will take a considerable amount of hammer. The 19J engines were prone to failure be it cracked piston crowns or heavily worn bores due to the unconsidered impact that the turbo had on what was quite an old designed cylinder block (based on the earlier 1950's engines) and most will eventually run on their own oil and destroy them selves in some shape or fashion.
If you're only going to be using it as a casual drive then I'm sure that it will be alright no matter what engine you have fitted, but if (and when) you decide you like it so much that it becomes your every day vehicle, you are really going to want a 200Tdi engine sat under your bonnet - inquire as to how much it might cost to source / have one fitted.
Other than that, use your common sense - you're a mechanical engineer, so you should be able to identify any drive train issues or structural weaknesses etc. Don't be afraid to give the chassis a bit of a bang with something to check that it doesn't crumble away or puncture straight through.
If you do decide to go through with the sale then prepare yourself for years of tinkering, most of which is required on a fortnightly basis. Expect to pay high prices for parts and components where you live - parts do seem to be scarce anywhere outside Europe and any second hand parts you do get hold of are more than likely taken off the thousands of Land Rovers stolen over here and shipped into Africa / Asia every year.
-Pos
