If the question's what will you have
to change, then the answer is not much. If you want to run dirt tyres, you can mess about trying to fit two rear tyres, but for the sake of a pair of secondhand S or E model wheels, you're up and running with proper dirt wheels. There is a reason dirt bikes have 21 inch front wheels after all.
If you want the original speedo to read correctly, you then also want an S-model speedo drive. Again, secondhand is just fine.
After that it's up to you. Personally, for my current bike, the idea of down-grading the forks in order to fit a smaller front disc was laughable, and I run the SM forks, and both SM discs - MUCH better braking on tarmac, particularly noticeable when laden. If your trip is going to be all off-road with practically no luggage, then fine - but even so, the chances of damaging the SM front rotor are pretty low when travelling (as opposed to ragging round some woods in an enduro event or what have you) - you can slow down and take a careful route through any given obstacle. Remember the DRZ S-model suspension is generally figured to be a bit on the soft side, add a bit of luggage and that'll only get worse. The SM kit I think will work just fine.
Gearing wise, the SM sprockets of 15-41 feel a bit tall with an 18 inch rear, I am currently running (S model standard gearing) 15-44.
I did loads of stuff to my bike
, but that's just me, and just cos I wanted to. I'm really pleased with it after its 5k miles of travel so far, anyway.
'02 Africa Twin (sold), GSX-R 1000 K5 (sold), '97 TL1000S, '08 DRZ400 SM/S, '92 CRM250