I dont think there is any vintage Omega bigger than 36 mm that I know of, in the dress style of the seamaster 600 . I can only suggest maybe looking at 50s models 30 T ,265 cal ,266 cal ect ect ...
IMOP its all about how the watch looks on the wrist , as an example I have a Seamaster 30 only a 30 mm movement but it wears bigger than my 58 Seamaster Auto !
You really need to go to a shop or watch fair that has a few different case ref that you can try on. Concerning price its all about the condition. You will always pay more for a collector grade ,really good condition original dial watch , than one with blunt case from too much polishing and a refinished dial with a non original movement.
You really need to research the hell out of it so you know what your looking at . Are you in the Uk ? as there is another watch fair coming up at the start of Dec , there will be some of the sellers at the fair who I met at this weekends fair, plenty of vintage omegas, longines and it would be a good opportunity for you to try out some watches all in one place!