My girlfriend and I are currently riding DRZ400s from London to Australia, and so far they have been a great choice. They will happily do 100km/h on a motorway, although not too much more! They are comparatively light.
Couple of things: seat comfort is poor. Really poor. We have overcome this by buying a Thermarest Lite seat and having an upholstery shop in Turkey stitch a little pocket so we can strap it over the seat. It improved things massively! Otherwise get an upholsterer to widen it and maybe put some nicer foam in.
We are using Peli 1430 cases on a Hepco & Becker rack with home made mountings. These are great cases, but quite small (18 litres). So we've ended up with a little too much in a bigger bag that sits on top. Makes the bikes a bit top heavy, and would be a bit of a handful on proper off-road. Pack really light! There's a guy who does Africa trips on a DRZ400 and saws the handle off his toothbrush. For a good handling bike probably worth the extra effort keeping the weight down.
Finally, don't be surprised when they use a lot of Oil. At high rpm the oil apparently blows out of the crankcases and is burnt back through the airbox. Ours will use a litre or so between oil changes. Check it frequently.
If you do an internet search you will find quite a bit of info on other people's prep. There is one girl who was riding one around South America and put a good write-up on prep on her web page. Pumpernickel on tour or something similar.
Finally, for a bit of inspiration on the joys of light weight motorcycling around the world get a copy of Mondo Enduro. DR350s not DRZs but well and truly establishes it can be done!!