We usually do nitrox. It's $100 more for unlimited tanks at most places. Doing 4 to 6 dives a day and the bottoms times we normally do, it's not a bad idea. On the other hand, my profiles are pretty conservative even without nitrox. With only a few exceptions (Hilma Hooker -100ft wreck and Angle City-80 ft double reef) do I go over 35ft deep, most of the pretty stuff is shallow and I want bottom time. I also spend lots of time in the shallows- 15 to 20 ft, doing extended safety stops and honestly burning down the tank, I hate to waste the air. I rarely leave a site with more than 400 psi. Considering I am in 15 ft of water and 50 ft from shore, it's not unsafe. There is a lot of life to be seen there if you stop and take the time to look. A lot of my better pictures come at the end of the dive in 15 ft of water. I know there is no evidence that nitrox makes you feel any better but true or not I do seem to be less tired diving it.
Your wife may or may not need a new computer depending on how you decide to dive the nitrox. We typically dive it using air tables/computers. My wifes computer is air only and I dive 2, one set to nitrox and the other to air. The only time I recall any of our computers getting close to NDL was during a deeper dive (the Hooker). Our air computers were getting close to NDL but the ones set on nitrox were no where close and once we accended to the reef ledge at 35 ft the computers come well below the NDL. If you keep your depths to 40 ft or so, it's real hard to push the NDL of air much less nitrox, tank volume becomes the real limiting factor. MOD is not a big problem because all the sites I go to are 100 ft or less.