For any 4x4 box that is strong you are going to spend over £3000, it is likely to make make more noise although I quite like the noises that come from my Oppliger box, mine is close ratio and by the time I sourced a decent casing and got Bara to build it I was in at about £4,500 with the updated vc etc.

Converting to 2wd will probably harm the resale value of the car I would have thought being non original layout.

Any box will cost a bit if you want the peace of mind of a rebuilt box.