Absolutely you can mix and match 1st / 2nd stages as long as you set the intermediate pressure to what the 2nd stage wants.
That said if you already have a set you can just get it swapped over from yoke > DIN.
https://www.amazon.com/Scubapro-Din-Conversion-Kit-Universal/dp/B0000DYVG3
While it is true the tech hasn't changed much that doesn't mean older stuff has lots of value.
Take something like an Aqualung Titan that is easy to find for around $400 and figure out the value of it used. To start with it is recommended to get it serviced which works out around $150 ($75 parts / $75 labor) so at most those regs are down to $250 in value to match what it takes to get them in working order but nobody is going to pay the same price for a used set of regs when they can go buy new for the same price so you need to knock some cash off the price to be competitive.
You are now down to around $100-200 of value on the used reg market. change to a $1000ish Apex DST/XTX50 and you are looking at $300-500 in value.
General guideline for used regs is expect to pay25-50% that of the set new if it is still possible to get service kits for them.
I get Dry mouth when Diving, I used to take my regulator out and rinse my mouth out with water and spit it back out (diving in fresh water).
later I started using an Apollo Bio Filter, this little device adds moisture back into the the compressed air (all moisture is removed during the compression process) and this made diving much more enjoyable.
https://www.amazon.com/Apollo-Bio-Filter-Moisture-Regulator-Authorized/dp/B07CKS7KB4
The body of the DIN and yoke first stages are essentially the same. The only difference being section that attaches to the tank. For what you are wanting to do it is probably easier to buy a yoke first stage and then get the corresponding DIN conversion kit. That way you will have all the parts for both.
For some reason the manufacturers seem to expect you to switch from yoke to DIN and not the other way so there only seem to be DIN kits. Just make sure there is a kit available for the first stage you choose.
It is a pretty straight forward swap out with some basic tools but if you aren't comfortable doing it yourself any LDS that does servicing should be able to help.
Example:
I believe this is what I need to purchase, Just curious if it needs to be done by someone or if I can do it myself
This video perfectly describes why I will never kayak.
The only time I would ever consider it is if I had a pony bottle, guaranteeing my safety if I were to begin drowning. In fact, I'm curious as to why small air tanks aren't in the recommended kit for rapids.
For those curious: http://www.amazon.com/DXDIVER-Bailout-Regulator-Secondary-Egressor/dp/B00TXN1EVY/ref=sr_1_2?ie=UTF8&qid=1437607236&sr=8-2&keywords=Pony+bottle
> I do think there should at least be a competency test to own any firearm. I need to pass competency tests to buy a regulator for scuba diving
I don't see where to enter my test results on this Amazon.com page, and I'm sure if I win any of these eBay auctions my regulator will come to my door by UPS.
>to be able to operate my car
Only on public roads. You can buy a car, never register it, never get a driver's license, and so long as you drive it on your own private property, no law has been broken. You can even let your 13 year old drive it.
>Shouldn't we at least require, or make incentives for being a gun owner that has demonstrated that they will own and maintain the weapon properly?
No, if for no other reason than because a testing system will always be turned into a means of denying our rights by inserting artificial barriers to passage. There can be no reasonable test to qualify for exercise of rights. They used to have poll taxes and literacy tests to keep people from voting. Unconstitutional.