I have a 644DS, and a couple of mates have the 404DS. As you've already found, the CCM Riders site is a font of knowledge and the few problems there are have all well documented on there.
As you've also already found the quality of finish is not the best in the world, spoke nipples and frame finish are particularly appalling. The other weak area is electrics. I don't know whether the wrong farme dimensions were sent to the loom maker or CCM were just tight, but there just doesn't seem to be enough wire in places.
Two common failure points on the 404 are up by the headstock and behind the side panel cover over the frame rail. The wiring can chafe through at both these points cutting the electrics, so worth a look.
Also worth looking at the sidestand bracket, I've heard of 3 or 4 recently snapping of the frame without warning (including my mates) so worth a close inspection of the weld at that point. My mate had just got off his bike at work and was walking away when it just broke of asnd his bike toppled over onto the floor.
Off road wheels sets come up fairly often on ebay and fetch about £200/250 so worth having, but make sure they come with longer sidestand and the shorter brake caliper bracket as buying seperately bumps up the price as these two items are much harder to find seperately.
DRZ engine is generally reliable, weak points being the very thin magnesium engine cases which can hole very easily in a topple over as the levers run so close. It's worth radiusing the shap point off the back of the gear lever to help. CNC machined bond on protection plates are cheaply available (about a tenner I think) on ebay, and worth fitting for peace of mind.
The other two are the gearbox output shaft spacer corrodes and leaks oil, again stainless ones cheaply available on ebay, and the timing chain tensioners can fail, obvious as engine goes very noisy and clattery, but once again, aftermarket manual tensioners cheaply available on ebay.
Otherwise, great fun, cheap to buy and vastly under-rated. Suzuki engine, quality brakes and suspension, English frame - apart from the crap finish, what's not to like.
Enjoy