Might be worth asking Mark at Marlin the differences in the chassis. Looking at mine, I have the brackets form the gearbox mount for the K as well.
600 TI looks a good choice. They usually have wasted spark ignition as well. No dissy cap. I think for reliability.
These have hydro clutches though. Good for you but needs extra work and a master cylinder at the front. Can be converted to cable clutch but looking back I wish I had gone with the extra work and gone hydro.
I think the hubs, brakes calipers, heater and instrument cluster will all be different on the 600 though.
Might be worth checking on www.rovertech.net/forum
If I was at the beginning and at a stage of making a choice, I would be considering Tyre R engine.