Even despite Scubapro's efforts to control parts supplies I'd still consider a Mk 2 Plus R190 or R390 to be the ultimate in simple to service regs.
There are an absolute minimum of parts, no adjustments, very few specialty tools needed and there is only one moving part and two dynamic o-rngs in the first stage, both of which are only subject to intermediate pressure. So it is nearly impossible to screw one up.
Similarly, the second stage has very few parts and a simple downstream poppet design. The only part that usually needs replaced is the soft seat and this can be replaced by removing the inlet body to expose the seat without removing or dissassembling the poppet itself, which in turn means no need to readjust the lever height. That leaves you only with a relatively simple orifice adjustment.
And in truth, parts are not a problem as parts are regularly available on e-bay. On any given day you will most likely see parts kits for the Mk 2, Mk 10, Mk 20, Mk25, R190 and G250 first and second stages. SP uses a very small number of kits for a large number of regulators. When you combine this with the number of SP regs out there, the demand for parts, and the number of offshore dealers, former dealers and liquidating shops meeting that demand, parts supplies are better for SP regs than any other brand despite SP's efforts to control parts supplies.
Specialty tool wise, you only need a multi wrench, an inline adjuster (with IP gauge), a set of o-ring picks, a yoke nut socket (or allen head socket if you have a DIN connection) and a torque wrench (not really a specialty tool, but important and less than $50.00 at the average Ace Hardware).
All of the tools required as well as an excellent book on reg repair are available from
www.scubatools.com
