people ask me all the time for bike buying advice. my standard answer is to get something that fits you. if you get a bike that isn’t comfortable or hurts you somehow, you won’t have any incentive to ride. for a long time, there was this notion that saddles and bikes in general would be painful and need “breaking in,” especially old racing bikes. we’ve come a long way.
if you want to buy new, check out local shops to try different styles. with your back issues, a more upright hybrid or city bike style would probably work well. make sure you take different brands/models on test rides to see what feels good to you. after you have some idea, you can either look for a used bike or go back to the shops. but you really should do some homework first. it’s a highly personal decision.
the nice thing about cycling is that it’s a low impact activity. there are people in their 50s and 60s who can destroy me on group rides, and i’m an okay cyclist. it’s possible to remain active and viable for a long time in cycling.
another thing to consider is a recumbent bike, which may be better for your back, but will not be easy on your wallet. most shops don’t carry them, so look around to find local dealers to check them out. they’re pricey but work very well for those who need them.