I'm not sure a DRZ would be my first choice for what you have described. If you're riding 15K per year, you're doing a lot of street and I assume a decent amount of highway miles. The DRZ really starts to suck as you get above 60mph. Moreover, it seems when I had mine and spent time on Thumpertalk, there were more than a few DRZs getting rebuilds at 20K or so. That may just be how they were ridden/treated, but i don't think the DRZ shares the DR's level of durability.
I'm not trying to imply a DRZ only lasts 20k or that it can't be ridden on longer trips - there are plenty of people that have done just that. I just don't think it is the best bike for it. I think a DR650 would be the better choice, but you already own a KLR so that's sort of a wash. On the lighter side, I personally would try the WR250 before I would go back to the DRZ. given that I went from a DRZ to an X-Challenge, it should be obvious how I think they compare. But mods aren't as cheap or plentiful and sometimes you need to fab your own. Although it sounds like that is your thing. But if you don't like the airshock, you're looking at $600+ for a decent aftermarket replacement. Not to mention $4500-5000 for the bike itself.
Maybe an older 640 Adventure is in your price range. I don't know much about them, but other than some of them being vibey (hit more miss), I was under the impression the LC4 engine is fairly durable with low maintenance intervals. I could be wrong on that though.