The 105K mile belt change/Service isn't an issue.
Firstly. All the internet scaremongering is WRONG. The rear (fuel pump) belt can be done without lifting the body, and is a 1hr job for anyone with a bit of common sense who's prepared to put up with a little discomfort.
I've done mine - Genuine LR parts were @£50.
The Timing belt is also part of the 105K service, but again it's very achievable by the DIY mechanic.
I'm about to do mine in the next few weeks, genuine LR parts are @£70.
LR charge @£650 to do them with fluid changes etc, so if you can't tackle it yourself and are buying a higher milage D3 make sure it's been done.
There are loads of other things that can go wrong, but are fixable.
So my advise is not to buy an older/higher milage example if you can't fix it yourself, and can't afford to pay LR £100/hr to fix it.
The HSE does weigh in at 2.7tns, the slightly less 'toyed up' examples are less, but you wouldn't know it when driving one, and I think it makes it a great tow car.
My HSE does @29mpg on a motorway run while not hanging around, or @32mpg at 70mph. Around town 23mpg is usual, I won't even mention what that drops to when towing.
As mentioned DISCO3.CO.UK - Index
is where it all happens.
If I had the money I'd upgrade to a D4 in an instant, but the difference in price between a used D3 and D4 is huge.
Mine will probably have to go when the RIB gets sold, and I'll almost be as gutted to see it go as the RIB.