Are you riding your 70's vintage bike right now? I am riding an early 80's bike. For my current needs, it is probably the best thing for me, except for the low handlebars. I am not very flexible, and and thinking about swapping out the handlebars. I am riding on a paved bike lane with very low traffic, on flat terrain.
I haven't ridden the bike since the mid 80's. I asked at the first bike shop about putting different handle bars on the bike, and was basically told it wasn't worth it. I'm sure the bike needs a lot of things to bring it back to riding condtion? At least it's been stored inside the garage entire time.
Remember, your doing this for FUN, so quit stressing

. And don't feel pressured into buying a bike. Take it for granted that there is probably not one 'perfect' bike for you. Just buy what is best for the needs and style of riding you anticipate right now. And enjoy the process

.
Mary