The OE or AM locker debate continues to rumble on, but I will give my 2 cents worth anyway.
If the factory lockers are used often then they will engage as soon as the dogs line up, 1 -2 seconds if that on the front (steering allows the dogs to rotate and align quicker), rear 4 -5 seconds, centre is immediate! I have OE factory lockers, they are brilliant, they were slow to engage when I got the car so guessed thy were not used very often? A few figure 8's on the beach and they come back to life and now respond to the switch pretty much straight away. I don't think you are going to break an OE locker with stock wheels/tyres, and would guess you are going to have to be pretty hard on the car even with bigger tyres. Breakages normally come from asking the locker to perform beyond it's design parameter. Mine are now 21 years old and have never had a spanner near them, and work fine.
So to aftermarket, I had ARB's front and rear in my competition Land Rover Discovery (see avatar), it was worked mercilessly and I never broke an ARB but, there was air leaks from the inner seal meaning the diff had to come out, then there was oil blowing out from the activation solenoids and so on. I accepted a lot of this because the vehicle was used hard, but on reflection when I consider the 80 see's some pretty serious off road work (not competitions), the OE stuff simply does not need any fettling, I just forget they are there until needed.
The Discovery was sold a few years back now, so the minor problems with air leaks may have been sorted? You also get on board air as well, don't underestimate how important that is.
IMO AND IME, do not pass up a mint 80 if it has no lockers, they are getting rare and good ones are thin on the ground. I would go for condition over lockers any day. The money you spend on getting a scruffy with locker example up to scratch you could have put it towards a set of ARB's, or, go for OE install, they are about second hand, you can strip clean and install with the minimum of effort, wiring (or most of it) should be there, now you have a 'minter' with lockers, that would be my route.
regards
Dave