I don't think a 300 Tdi engine will fit straight in, also i don't belive the LT77 gearbox will fit to a 300 Tdi without an adaptor kit. I have heard of Turner Engineering, one of the Landy mags uses them. The price for the 300 Tdi is for a stripped engine, you would need manifolds, turbo and all the other bits that bolt onto the engine. The 200 Tdi would be a better bet. As the price for this is for an exchange unit you would need to buy an engine to give them, but it should have all the bits on it that you need. Either way it would cost. If you definatly want to change the engine a re-manufactured one would be the best choice,if you just buy an engine from someone you might just inherit different problems. All this is cost permiting.
As for you current engine going, really there is no way to tell. Mine is on 173000 miles and working fine, others go at 100k miles. I have not heard of any of these engines giving out because of the oil/air filter issue. The main issue is that the engine is quite highly thermally stressed. Land Rover did not at the time have a powerfull diesel engine to compete with the Japanese 4x4's. They did do some work to improve the engine, but it is basicly the old 2.5 diesel with a turbo bolted on. No intercooler was ever fitted.
If you look after the engine and don't give it a hard time it should not fail. Make sure the cambelt is changed at regular intervals, book say's 6 years or 60k whick every is sooner, I change mine at 50k. Change the oil at the right times, keep an eye on the cooling system. All routine stuff.
I don't drive mine at full throttle on the motorways, it is happy cruising at 60 - 65 mph. It will go faster but it heats the engine up quickly. I do use full revs, but do not hold it there, just for accelerating throught the gears.
The best idea really is think long and hard before spending out.