Hi Gaz and welcome to the forum
It really is down to personal choice, as both the Rover V8 and the TDi engines have been around for donkeys years and have massive parts and supply backups. I cant comment on parts prices though as Ive never owned either of them myself.
if you are looking for a pure toy then the V8 would be my first stop, for the noise and power. However, the big drawback is that if you are planning to offroad it, petrol engines really dont like wading!!!!
as for purchase and running costs, the lower price of a V8 (due to fuel prices putting people off) would probably be balanced by having a professional LPG kit fitted if there isnt one; if there is then there will be a premium on the purchase price anyway. You have to think about the practicality though as there isnt much room for an LPG tank in the back of a 90 if you still want some space for loads/passengers, but underslung tanks are generally a big no-no for off roading.
As far as running it is concerned, then a 300TDi will quite happily run on anything up to 40% straight veg oil mixed with the DERV in the tank, making it much cheaper to run than usual.
Both engines will go on for a long time if well maintained but both have major issues to check for. The TDi needs the cambelt replacing religiously at 70k mile intervals, and if the vendor cant produce proof of when it was last done, budget for it to be done ASAP. if it goes, wave goodbye to most of the top-end valvetrain.
The V8 likes VERY regular oil changes, without them it will quickly sludge up inside and die. If the oil is black and thick in one youre looking to buy, walk away. Also, while most people go for a carb'd 3.5 or an EFI 3.9, be wary of the 4.2 and 4.6 litre versions - they are known for porous blocks and, especially in the case of the 4.6, cracking between the cylinder and coolant jacket, letting coolant into the cylinder. This is a new-engine scenario.
Whichever one you choose, buy wisely and youll have endless fun!
Hope this helps,
1982 Series 3 88" County Station Wagon - needs new bulkhead, swivels, unleaded head, wiring
2000 Discovery 2 TD5 ES - new daily drive
1910/1994 Marshall traction engine - 1/4 size