@ Randy and @ Vaughn
Hey friends, that are really helpful answers.
I´m thinking to go 5x7 with my Sinar because the difference of the rear frame holder is not to big compared to 4x5. And cheaper too. The format adapter plus the complete back incl. groundglass is not very expensive today in used condition. I have to look for a not very contrasty film too - I think there is some to buy. A 200 ISO type film should be the right one.
I´m glad that someone else let process the x-ray film at the doctors lab too. BTW, the nurse isn´t really good looking but that fact is not very important.
Thank you for helping with X-ray photography.