The most important thing is that the motors are isolated. A good clue to this is if there are two wires coming from the motor. Neither terminal on the motor can be allowed to contact the frame (and thus one rail) directly. If the only path is through the wires, then adding a decoder is easy (electrically). If the motor is NOT isolated, you will need to devise an alternate method of mounting the motor so that it is insulated from the chassis before you can install a decoder. This is bascially a mechanical operation, and can vary from pretty easy to "tear apart the whole drivetrain and rebuild it". Never fear, most of the brands you mentioend should be already isolated, or easy to do. It's the old open frame motors with all-metal chassis and all metal drive trains that can be trouble.
Assuming you have an isolated motor, the next hardest thing is where to put the decoder. Usually the boiler is filled with a weight, so the tender becomes the logical place. If one side of the pickup if through the tender wheels only, this actually means you'll only have to run 3 wires up to the loco. If there are pickups on both sides of botht he loco and tender, you'll need 4 wires - also if all pickup is in the loco itself. A posisble oddball - some of the Mehano locos are driven by the tender, and the loco has no motor. That's basically like installing a decoder in a diesel.
A nice low cost way to start adding decoders is to use the NCE D13SRJ. Empire Northern Models has them for $119.95 for a 10 pack - less than $12 each. Hard to beat, and it's a very good decoder, not some cut-rate cheapo. You can also buy 4 packs and singles, but that drives the price up over $12 each. Still a great deal.
--Randy
