Hi Sid... I'm late 40s and also fighting back from injury to get my sorry behind back into shape.
I've consulted with my doctor, my naturopath, my CrossFit trainer and the good ol' interwebs (PoliquinGroup.com helped) to come up with this batch of stuff:
#1.. EAT RIGHT AND GET ENOUGH WATER! I put that in caps because it was yelled at me. Get enough fruits and veggies, lean protein sources, enough fiber, no sugar or processed crap, and keep hydrated. I'm trying to eat Paleo and it's helping. If I eat garbage and don't stay hydrated, I notice this right now.
#2.. A good multivitamin. All the Super-Bison-Testo-Max in the GNC store won't help if the basic building blocks for our system aren't there. Good ones are still cheap. Vitamin D is really vital in this category, especially for us folks over 40-ish.
#3.. A good protein supplement. You can get a five pound bucket of good whey protein nowadays for reasonable coin. It's hard to get enough quality protein, and a scoop in the morning smoothie is a real easy way to get a good dose. Protein is vital for gaining/keeping good lean muscle mass.
#4.. Essential Fatty Acids (EFAs). Look them up and you'll find Conjugated Lineolic Acid (CLA), fish oil, flax oil, omega-blah-blah-blah... and a million reasons why they are good to take in. A good quality EFA supplement is another inexpensive addition to the regimen.
#5.. Branched Chain Amino Acids (BCAAs). Again, I could write a dissertation on why these are a good add, but the interweb has done it for me. Take a google for yourself, and you'll see why I add these to my water bottle before CrossFit.
#6.. Creatine Monohydrate. Reams of legitimate research on why this stuff is good for adding muscle mass. I don't use it myself at the moment, but when it gets to the point where I'm lifting more than my sorry carcass in weight, I probably will. It's cheap enough to add.
I had my naturopath run a complete panel on me to dial in on my selection. She ran everything... food sensitivities, lipid panel, adrenal function, vitamin deficiencies... I'd suggest you do the same, just so you don't add something you don't need, or miss adding something you lack.
Good luck, Sid. I hope your journey back is fun and productive.