I have used spd (regular ones, mtn bike type) on my touring bike, commuter and cross bike (really used only on road) for about 20 yrs.
Recently got some new shoes--Specialized mtn ones with nice stiff soles-which I find great for comfort and pedal force transfer, and still being relatively easy to walk in. A plus for me.
Last summer my wife expressed an interest, just like you, to gain some efficiency and she went the same route as I have for shoes, but after using my old set of spd pedals, she got some spd on one side/flat on the other, and she likes them a lot.
I still see spd shoes that allow reasonable walking (there are tons of models out there that can take the cleats, from regular looking, easy walking shoes, to super stiff mtn bike shoes) combined with the reasonable cost of Shimano spd pedals (520 models cost about $50) and the fact that Shimano pedals are very well made and will last many many years--seems like the best option.
If you see in the future that you want to go whole hog roadie shoes and setup, then do it, but the spd setup can be done with very good quality shoes and pedals for $200 easily.
PS, as for shoes, try as many as you can and go with the one that fits the best and feels good right away. Arch support and fit for my narrow feet were my priority, and were worth spending $50 more for the shoes I got compared to others that were good but not perfect.
Really appreciate being able to read this conversation, even though it is quite old. I bought Shimano EH-500 pedals (SPD on one side, flat on the other) for my road bike, that I have been using with EX-300 shoes without the cleats attached, except when it was below 40* recently, when I wore my flat bottomed street shoes. But as I'm getting better at biking, I want to return to the efficiency I had when I wore toe clips in the 80's. I won't be able to ride for a few days with our high winds and rain (fairweather cyclist and golfer here). But I did install the cleats on my shoes today - my only cycling action at all. Maybe tomorrow when the wind isn't howling in the garage, I'll practice clipping in and out. I'm planning to only clip in on one side for a while to make sure I'm used to it. I'd like to be one of those people who has never fallen over. It never happened with toe clips.