Scubapro (and many other) regulators are manufactured to their specs by a company called Tabata. Scubapro designs a regulator, sends them the design, and then have some amount of time with exclusive rights to that design. Often they license older designs to various companies, one common one is TUSA which stands for tabata U.S.A. Since they've already been making the designs, it's very easy for them to change a few cosmetic things and build a more-or-less identical reg and sell it themselves. I'm sure the Halcyon regs are licensed copies of the MK25 G250. Scubapro no longer sells the G250, one of the more recent in a line of bad decisions by the company.
Getting back to the OP, if you want a MK25/S600 but don't want to pay a high price, get a used one. They're all over the place, those are very durable and trouble free regulators with good parts availability and a lot of dealers with technicians of varying quality. The warranty of new regulators is a joke...you end up spending more in 'required' service than you do on the regulator, just to keep the stupid warranty in place.
Don't worry in the slightest about ebay rebuild kits being a few years old. EDPM o-rings have a very long shelf life, I've seen MK25 seats that must be over 10 years old that look and work perfectly, and those regulators only need to be rebuilt maybe after about 200 dives (maybe more) as long as they are cleaned well after salt water use and stored well. That's often 4-5 years for a typical recreational diver.
