I'm older than you are too...( and I hope your health issues can be improved a little - so you can get to the range ) ...
However, there are all kinds of guns out there on a budget...up to $ 800 or so out the door...from S&W, Springfield, Sig, Glock, etc...it really comes down to what fits your hands the best, what controls you like on the gun, what you like esthetically, etc...
If you like the M&P, you should buy that...
Personally, I have large hands - so all of the sub-compacts, most of the compacts do not fit my hands and I can't shoot them very well. So I tend to look at a full sized gun for my needs.../ but I say that only so you understand how I look at picking a gun ( first and foremost it has to fit my hands ! ).
The other reality is - the price of the gun, is probably the least expensive thing you'll buy in this hobby ....vs ammo, range expense, training, etc..../ ...and I would encourage you to get with buddies for a range trip or sign up for a class...discuss your health limitations with the instructor ..and find one that suits you / will work with what you want to accomplish.
I have gone to a 9mm for defense also ...as arthritis, etc has become a bigger issue for me too. Nothing wrong with a 9mm ---especially since there has been so much good "Defensive Ammo" developed for the 9mm in the last 10 yrs or so.
You should shoot / rent as many guns as you can ...before you buy. Consider single stacks and double stacks...lots of options out there today.