Landrovers were all designed by a team of qualified motor engineers, and usually work well if kept the way the designer intended, and serviced as he instructed.
Most of the problems I have seen in my 35 years of working on them were caused by owners who thought they knew better, and either bolted on non-standard items, or did not keep up with servicing and maintenance.
There are a very few exceptions.
A Tdi, 2.5petrol, or a V8 is an improvement on a 19j, if you want more power, and not worried about originality.
Axle lockers are a big improvement over standard in greasy conditions, and should have been fitted as standard.
And in many places, slightly more aggressive tyres may prove useful, although myself I would never fit larger tyres unless I intended to drive in competitions, and was going to upgrade a lot of other stuff as well.