All comments, above, hit a lot of good points...
But you haven't really said if you have a preferred budget.
You LIKE cycling, so the question is : Would you possibly be open to having more than 1 bike at some point? It means quite a bit in what you might start with.
I have a few too many bikes, but then it's been a par of my life for more than 58 years...
BUT, today, If I could have ONLY ONE bike - it would prolly be a 'Gravel Bike'.
It's not just for 'gravel'. Gravel bikes are just adapted road designs. Adapted to be able to ride just about any surface you might encounter, and just any hill...
As bikes go, they can be just as nice on a paved road as on a dirt road. A lot depends on the tires you decide to use.
So you might consider more of what you might wish in a bike.
I recommend you visit the Gravel/Cyclocross forum here on BF and look at the images of bikes riders have. They go all the way from very dedicated 'offroad' riding to bikes which could easily be confused with Top Line Road/race bikes... A great variety
I could be very happy with just ONE gravel bike setup for my predominant 'roadie' core and now my more often riding a gravel bike on what I call MountainBike-Lite terrain....
Luckily, I have no problems with having many bikes, so I can choose the bike I want to use, depending on what I wish a 'ride' to be.
but having just one bike is very OK also.
...worth considering...
and cycling can certainly be a good component of dealing with the health issues you wish to address.
Ride On Yuri

Welcome to BF, cycling a wonderful part of a lifestyle