Although you can use RR or Disco parts the correct 90/110 items are a direct replacement and fit perfectly (I didn't even have to drill any mounting holes for the reservoir whan I did mine as they were already there). Whan I fitted PAS to my old 2.5NA D I used a complete kit off a later Defender and it went straight on (bottom pully, pump mounting bracket etc). Bought for £150 at Billing, I had the whole job done in 4-5 hours (fitted new steering column UJ's at the same time as it was easier to do it then). Can't think whay you would need a new drag link unless that was because you used Disco parts? 4-Bolt box or 6-bolt (wasn't there a 3-bolt as well?), they all fit but the 4-bolt is the most common (it is the bolts on the top of the box that are counted, not the mounting bolts).
I am sure everyone knows (it even says it in the manual) but never hold the steering on full lock for more than about 20-seconds, LR PAS is known for being weak and doing that can lead to rapid failure.
When I had a new 300TDi engine & ancilarries fitted into my 90 I decided to fit a new box at the same time (everything else was new so why not that?), recon boxes are ok but there are some people who fit a new bottom seal to an old box, paint it black and sell it as recon. To avoid the chances of this I went straight for a new (genuine) box, be interesting to see how long it actually lasts (the old box was fine when I removed it and sold it on).