"What are things to look out for on older 1970's CB750's (US)Paticuarly 1975"
Other drivers, especially in SUVs yelling at the kids in the back while talking on the cell phone....
As for parts, I've seen tons around for the earlier 750s (69-76). Here's a link for some specs on the differences between engine years. http://www.hondachopper.com/engine.html