Nothing at all wrong with older equipment, and your right, pretty much nothing of value has changed in scuba regs or BCs in the last 20 or more years. As long as it's a US Diver or Scubapro odds are parts are not a problem. If it's a MK-10 first stage then you have a good solid reg and you can still get service parts for it, same goes with any SP second stage that was originally attached to it. If it happens to have a different brand second stage or octo, you may have service issues but most can be serviced. Worse case you buy a SP octo off ebay and have it serviced or buy a new second stage like a HOG, Dive Rite or a host of others and use it as you primary.
The BC is no big deal. Inflate it fully orally and see if it stays (mostly) inflated for a couple of hours and all the inflate/deflate controls work, if they do, you are good to go. If it drops pressure a little, no big deal and even if it does deflate quickly before chucking the idea, look for the leak. Odds are it's a sticking dump valve or inflator. Inflators can easily be rebuilt or replaced and 90% of the time a leaking dump valve just has some grit in it. As long as it's not leaking somewhere in the bladder, everything else is easy to repair.
Don't get caught up in the "how much is your life worth" or "it's life support equipment so you have to have new or you will die" noise. This is from folks who have spent way too much time listening to uninformed dive "pros" or shop folks wanting to sell gear. Reg designs have changed little internally in the last 30 to 40 years. Sure the external features are different but the interal working parts have pretty much stayed the same. When a service is done properly, all the internal parts that age or wear are replaced and all that remains from the original reg are solid brass and stainless steel parts. These parts have pretty much an unlimited lifetime, I have plenty of 40+ yo regs that are in perfect working order and there is no sign that they will ever not be servicable as long as I can get parts....Come to think of it, the only regs I have had to take out of service due to damage were all modern plastic second stages.