Ask nicely on here to see if anyone will go with you to see something. I find that MOT's and their advisories don't really mean a great deal, things can be missed, things can be made to seem worse than they actually are. The proof is really in the car itself.
300Tdi's are a good engine, parts are easy to get so this won't be an issue. I personally prefer the 200Tdi, but parts are going to get harder to get soon enough. The last 200Tdi was done in 1994(ish) so that is 20 years ago. the 300Tdi only a few years newer but as the military still use them you should be able to get parts for a long time yet.
For an initial cost perspective you will get more for your money with a Discovery than a Defender. The Defender is very much like a meccano set, everything is bolt together so if you damage a wing, unbolt it, bolt on another one. If you have rust in the bulkhead, unbolt the old and bolt in the new (there is a little more to it than that, but you get the idea). On a Discovery if you have to deal with rust (and you will) things are not as 'bolt together' as the Defender. But they are fundamentally the same underneath and with a Discovery being half the price of a Defender of the same age, you can't really argue.
And if you have to pay a garage to do the repairs for you then it doesn't really matter what you get, the cost of repairs and maintenance will be more or less the same.