I'm a Landy nut - ran quite a few over the years from Series III to a Disco but currently running a 1986 ex-military Ninety and a slightly more comfortable 1998 110 CSW.
You have a very fair budget, a 110 is far more practical for general family/utility/boat use. 90's are tiny inside and if you stick the kids/family/friends in the back then you have zero luggage space. I also find the 110 rides much better. There is no real advantage to a 90 other than a turning circle less than the 110 - comparable to an oil tanker...
 - for real off road use a 90 is more managable of course - talking trials/pay and play days etc - nothing beats a soft top ex-mod 90 in that situation for fun.
For many enthusiasts (me included) the last great engine was the 300tdi which is why I run a '98 110 - last year they made them. They go for a premium over a slightly later Td5 for this reason. You have plenty of money for a mint Td5 or a Ford engined 110. Problem with the latest ones is the seats - the old CSW with the benches (the '12 seater') has loads of people/gear space - the newer models with forward facings seats are far less practical.
Head over to some of the Landy forums for lots more opinions (Difflock is my favourite).