I'm sure you'll find a decent low miler for that money, again for some reason the mileage seems to affect the price as well, mine's done over 100k now and it'll go on for another 100,000 miles without much trouble no doubt (fingers crossed). You might be restricting yourself a little with the miles though because these cars are such a good long distance car most have done in excess of 60,000 but again they seem to take the miles a lot better than the P38's.
The tunit boxes shouldn't affect warranties as long as you take it off before you go to the garage (I hope not because I didn't bother taking mine off yesterday when it went in for the recall! - Mines well out of manufacturers warranty anyway as its over 6 years old). You can fit and unfit them in around 5 mins once you know what you're doing, I recommend them highly, had them on 3 of my cars now and once you've bought one its not too costly to swap to a new unit (Mine was £50 exchange with one fitted to my girlfriends old Audi A3).
Happy hunting anyway, can't wait to get mine back from landrover, they gave me a brand new freelander as a courtesy car which is a good car but just not in the same league as the L332's.