As mentioned, you really want to find somebody with ARB install experience to do the ARB. There are seals that are easily nicked and can cause problems down the road. Apparently most ARB problems are install related. When I did mine a few years ago I decided it was best to have the diffs set up by somebody with experience (I used Randy's ring and pinion - but I'm sure there are other places who would be fine). If you are going to pay somebody else to do the setup work anyway, it might as well be a shop that has experience with the setup you want. Use a shop that ARB recommends, that way in the unlikely event you have problems ARB and the shop can sort out if it was a product or install problem.
By the time you buy used diffs, buy gears, bearings, lockers, etc and pay for setup you don't end up saving much if anything by trying to do it locally - and it is not likely the local place has the experience with ARB that the bigger shops have (of course that depends on where you live). Add up the costs and compare for yourself. I decided it was worth an extra $100 or so to have the diffs setup by a shop with experience, and not have to deal with trying to resell my used diffs - just put them back in the box and send them back for a core charge refund. I put a v6 third in the rear - they are a little stronger - maybe not needed but the difference was small. I returned my 4cyl rear (which was a 2 pinion in my truck) to Randy's with no problem (I checked with them first)

If you live where there is mud and water extend your diff breathers - you don't want mud to get in and damage your expensive diff.