How many miles is too many? Well if it is a Series 1/2/2A/3 (ie pre defender) Land Rover they only had 5 digit odds so it's a case of knowing how many times it's been round the clock...........
My 1996 TDi300 (last engine without lots of electronics), 79000 miles (average for privately owned and age but very low it what you are looking at is ex Utility Company or ex MOD)), County Hard Top (Van to you and me) converted to a Stationwagon - had two cambelt changes, new chassis 4 years ago - have been offered £7500 for it as apart from some scratches it is in great nick. There is a lot of over-priced dross out there, but there is also a huge choice. No need to put monster wheels on either, the standard size 16" AT's are pretty good and you should see about 60k miles from a set.......... Putting much bigger on means all sorts of suspension problems, and fitting conversion bits. BTW there is a cult following for old Land Rovers, they are never "on their way out" might be crude and uncomfortable to drive any distance but they are always in demand. I brought mine in 2002 for £10k, it had the £700 conversion to a SW, £4000 for supply/fitting of a galvanised chassis, all new braking components, all new suspension components and a hydraulic winch fitted for that price. Mine has never broken down, it did get the rear hubs drowned (I did that!) and once you have one you never stop spending money on them as there is always something else you want to fit. It is reckoned that like Rolls Royces 80% of production is still running, and they started making them in 1948!
For a surf wagon what you want is an ex-Electric company 130, you can get your boards safely locked up in the back where the 'lecy company stored ladders, but a 110 would do as you don't then have to climb on the roof (and they are high!) I used to carry two windsurfers on my Defender................ one got badly damaged when it got caught by the wind before I could get up there to tie it down and it flipped onto the edge of the roof rack. Land Rovers aren't keen on salt water, rots the chassis very quickly! If you have to have huge wheels then a Disco1 3.9 V8 EFi would be a cheaper buy than a Defender, fuel consumption is crap (petrol and mine one could get 25mpg fully loaded up on a long run, but normally 16-17 round town)but comfort is huge and you can stack stuff on the roof and not make the fuel consumption any worse.