Ovid, welcome to my world and AirGun Nation
I am far from new to airguns, but sounds we have several things in common. I have a bad back, need a right knee and have torn my right rotor cuff...getting older is not a breeze. I have tried break barrels, did not like the problems for me to cock most. Also, as with all air rifles, there are hold problems that arise for accuracy. Then I purchased my 1st underlever, TX200 MK3 which was a breath of fresh air. Besides being a beautiful air gun, very easy for me to cock and extremely accurate. 2 other under leavers followed closely behind. Could not get the PCP out of the back of my mind. I knew pumping was not an option for me, since I could not find a refill shop close, I had given up. Then, almost like magic, I ran into an old friend I had not seen in years. To preface that, the only Paintball shop in the area had closed several years ago. Well, come to find out, the old friend is the owner of the old Paintball shop and it is not closed, he just had to make it "Private" due to his insurance. He told me he goes by the Paintball shop about every afternoon when he gets off his paying job. Said shoot him a text and he would reply with what time to meet him at his shop,...PLUS it is less than 5 miles from me. He has a commercial 4500 psi 4 tank refill station and has been to several schools on high power air, compressors and air guns, both PCP and Paintball. So I would recommend, unless you wish to purchase a SCBA compressor , not a Scuba (huge difference, SCBA are the carbon fiber tanks firemen wear when in smoky conditions and most fill to 4500 PSI where scuba tanks only fill to 3000 psi (as I understand, I am no expert) and end up much heavier. Depending on the PCP and the pressure it shoots at, a Scuba would not help much where a SCBA tank, depending on size, can refill a PCP many times over especially if filling to 3000 psi. I would find my air source before going any further. Having a PCP gun does no good without high pressure air.
For me, after reading for several years, the Benjamin Marauder seemed the way to go as a start. You can pick up a new unmodified one in .177, .22 and .25 cal for as little as $399 new (refurbished) or $499 sealed box (looking in the right places can help find discounts especially now with Labor Day sales). The Marauder is a good platform to start with and can be highly modified to shoot smoother, faster, or with more power depending on your uses and desires. You will need a scope which if you are like me, will depend on how your eyes have aged. Then other accessories as time passes, pellets are a must have.
This hopefully will give you an idea how I went about the process. I am sure others will step up with their personal advise.
One last edit to check on, most fire stations fill their own SCBA tanks. You may wish to check with any Fire Departments in your area. SOME fill them for free, some have a small charge and other can or will not fill them for different reasons.
Hope his helps
Charles