I would suggest a 650 class instead for your riding honestly.
Particularly since you'd be leaving a bike that has about twice the stock HP of a stock DRZ, and almost twice the amount of all but the most highly modded DRZs. A DRZ completely maxed out, finely tuned has been known to stretch into 60HP, but even then will probably be closer to 55 unless one strictly follows Eddie Sisneros' guide since he is like THE guy to go to for DRZ modification (He's in CO).
Maybe for the cash, a nice newer fuel injected street legal WR450F would be more suited, but IDK how people like them on high speed highways. I ride my DRZ400s with a pipe and airbox with a stage 2 DJ kit, and I don't like to take it cruising over 65, but it will do higher for passing.
If you get a DR650, XR650, or similar and get yourself into some technical stuff, you may be wishing otherwise, but honestly even then the DRZ is a top heavy fucker that is just a bulky dirtbike and not THE BEST at technical spots compared to a WR450 or WR250.
So, ask yourself, "What kind of riding do I want to do, and what do I realistically see myself going to do?"
A 650 for just trails is going to be great, and I've seen people do unexpectedly well in tighter terrain, it just make take 1 or 2 more tries, or they may go a tiny bit slower.
Either way, look into good technical riding skills. They will help one master a stubborn bike. Here's some youtube channels/videos I'd recommend watching.
Rampart Rider DVD -- RawHyde Adventure's Off-Road Riding Clinic, Colorado. Good 1.5 hour clip on some techniques (like slow riding) that are great to practice.
Crosstraining, Endurocross, and Enduro riding techniques. A whole channel dedicated to learning these skills.
And, on that last link's main page (not the videos page I link to) you can see Enduro Kex and other good youtube channels they're subscribed to.