Something that quite surprised me - on a visit to GRS yesterday I picked up the very latest LGB catalogue, and given what we've all been saying about the obsolescent nature of the MTSIII system and how Marklin/LGB are pushing their new advanced MFX/CS3 system quite hard, I was surprised to see that the MTSIII is actually still listed in their current catalogue as a production item!
Well done for taking the plunge, Trevor, when your stuff arrives from ML we will do our best to help you through the joys of decoder installations and so on.....
The great thing is that as you already have three decoder-equipped locos anyway, you should (in theory) be able to simply set up your new system as soon as it arrives, straight from the box, and get it up and running immediately with your Spreewald, 2-6-2 and KoF; fiddling about with putting the decoders in the Schoema and the Stainzes can wait till you're familiar with how your new system works.
Just one more question at this point - when you bought your Spreewald, your Harz 2-6-2 and your yellow KoF - did any of them have any indication as to what DCC address they were each set to, in the form of any kind of owner-affixed sticker or number on the locos anywhere, or written on the boxes they came in? It's not a problem if they didn't, as you can either re-assign new numbers from your new control system or else you can find what they are currently set to by a process of trial and error (you've only got 22 possible addresses to run through).
LGB MTS-equipped locos will usually be set to address "03" as the factory default, but of course a previous owner may (and probably will) have changed them to suit his own loco roster. The yellow KoF, being from the MTS starter set, will have been supplied with a default address of either "02" or "04" - but again, of course a previous owner may have changed that.
Jon.