I think it really depends on what you think your requirements are - a scenario would be to design and build one bogie with secondary, and the other without, and then test them to see whether the secondary makes any difference. I would prefer to have secondary in 21mm gauge.
Penbits' system would work with Murphy Models CIE diesels since the design is effectively a cradle that wraps over the existing power bogie, and the rest of the drivetrain is generally unaffected.
Penbits don't do anything specific for CIE, and certainly not the Sulzer, which - being A1A+A1A - would be unlike anything they have produced before. That's why I have been looking at the Hymek bogie kit, which is the correct total wheelbase of 10'6, and wondering whether it might be modified to allow a central unpowered axle.
The only other alternative RTR 10'6" bogie is the Bachmann Warship. Both Heljan and Bachmann I have found to be mostly unhelpful in answering specific spares queries, mainly because I think they don't really want to help.
My preferred motor is invariably a big coreless one - something by Maxon or Faulhaber / Escap, which are fairly easily bought via eBay; do a search for a US-based seller called "microlocomotion", because he seems to deal in just about every size there is.
Gears, I don't know. I'd prefer not to have to use worms; I like low-resistance bevels and spurs, or contrate-and-pinion. You can buy the latter from Ultrascale, but their order waiting list is notoriously long, usually six months.
RP25-88 is not finer than P4; flanges are thicker, and the tyre overall width is wider at 2.2mm or more. P4 is usually in the range 1.85 to 2mm wide.
A Sulzer 101 wheel is supposed to be 37" or 37.5" - 3ft 1in or 3ft 1.5in, depending on whose information you believe! AGW produces disc wheels
It sounds like you've got a far better-equipped workshop than the vast majority of us! Do have a look and see if you can turn out a set of miniature bevel gears or contrate-and-pinion.