Hi, and welcome to SRP!

Yessir, a straight can most definitely be used for head shaving.
I do that myself every day.
It will take a toll on your razor, but a proper stropping will get it back to form again.
A number on how many shaves you will get out of each razor before honing is impossible to give, there are just too many factors that will affect that.
For me, I tend to need a honing about 1/3 as often or thereabouts.

I would recommend getting the face shaves down first, then go to the head shaves with a bit of experience and confidence.
Once you have that, it is an enjoyable and doable task that will be surprisingly easy to learn.