I'd go after the two things that can plague a series Rover.
#1 as already stated is to replace every oil sea on the thing. It won't stop leaking but you'll know they are as good as they are going to get.
#2 is to go through all your electrical connections and ground points now, in the comfort of your garage. I also install a 2nd ground strap from the engine as this adds a little redundancy and reliability for starting. A major weakness is the lack of fuses on a IIA. I'd either graft in a fuse box or do a Painless rewire if there are already serious issues. That harness is already 40 odd years old and if its been in AZ all its life the insulation is probably quite brittle. I melted the harness in my 101 and 88 because of shorts in old wiring.
For upgrades I'd keep it simple. Lights, alternator, Magnecor leads and an electronic dizzy.
Replace the rear 1/2 shafts and drive flanges while you deal with the diff.
That'll make it nice and reliable for light duty.
For mechanical I'd sell the Fairey and spend some cash on a Roverdrive. I thrashed my 88 up and down from Phx to Tucson 5 days a week for over 100K miles before it eventually died due to lubrication issues of my own doing. They are much more robust and quieter than a Fairey and well worth the money. A simple brake upgrade is to fit the 11" drums off a 6 pot as I don't think your 71 will have them. 235/85s work well on these trucks.
Other than that it is a series. It is slowish and loud and have fun.
My NADA 109. She's a 20ft Land Rover but lots, and lots of fun. I'm still building bits of her.
