I forget which forum most of the discussion of these locos took place, either "Product Reviews" or "Rolling Stock." Overall, the locomotive is a fantastic value for the dollar. It's got a very high level of detail and runs smoothly (if a touch fast). There are some quirks with it, but none insurmountable. The biggest one is the counterweights. The first run's counterweights were produced poorly, leading to a degree of slop that would bind the valve gear. Anyone who bought the first batch of Ks can get replacement counterweights from Bachmann, and all subsequent batches that have left China have the new counterweights installed. (Look for a white sticker on the outside cardboard box that says "Counterweights Replaced" or something to that ilk.)
Here's a link to some articles by MLS member Dave Goodson (aka "Curmudgeon") on making some simple changes to the K to improve it. Unfortunately you have to be a subscriber to Garden Railways to read them. I know Dave's posted these things elsewhere, but darned if I can find them right now. Hopefully he'll chime in here.
I'm not sure how familiar you are with DCC? A base system would require a motor decoder installed in the engine. There are several specific installs listed on the Bachmann website. If I recall correctly the QSI is the easiest, closest to plug n play, and is also a sound decoder. You also need a DCC system and power supply. Have you looked into DCC systems? If you go to the DCC forum you can search and find several threads discussing different systems. If you have specific questions on DCC that forum is also an excellent place to post them.
I bought one of these a couple of months ago and think it is great.Andrew at Garden Railway Centres supplied a new set of counterweights with it which I installed with no problems in under half an hour.I have installed a simple chuff system using magnets(scalextric model racing spares)and reed switches from an ancient IBM computer,real coal load and a few other minor mods.It runs like a dream but watch the overrun this is a very free running loco.
The only problem that I can see is that you will need some serious stock to pull with it.he he he.