the medical kit is the cheapest healing upgrade in the game and it's attached to your line infantry, of which you should have at least two or three.
Yes, it's annoying having to sit out two squad at once if you're trying to healing, but it's one of the most mobile heal in the game and you don't need to pay upkeep for it.
By comparison the ambulance takes up 4 pop and will cut into the final army count in the late game.
I find this pretty ok considering the ambulance now has aura heal always active.
That and I have tons of vehicle crew to just sit out and wait while I build an extra 2-3 shermans while I already have 3 of them.
By the way, does anyone know how much manpower is ticked off from the total for every 1 pop that you are using? Hard to keep track of the numbers since sometimes it's odd and sometimes it's even.