+1.First off, welcome to SMJ !!!
Second, it really depends on what you want to do with it. Street riding, racing, are you planning to do any offroading with it, etc... Where do you live and what are the local laws? If you share your background (experience) and what your plans are for the bike, we can help find a better match for you.
I have a couple in mind, which are the WR450F and KTM 525EXC. The WRF is bulletproof, from what I've heard and I'm sure Crash will be able to tell you all you need to know about them if/when he chimes in. The KTM's are fantastic but I am biased lol. The EXC can be picked up pretty cheap wherever you are, I believe, make good power, look great and yes, need regular oil and filter changes, etc. but serve you very well if you maintain them.
I wouldn't go for the DRZ personally, as I'm not a fan of the looks and it seems everyone who has one gets bored of the weight and lack of power after a few months.
Good luck with the search though and please do a build thread if you go through with it.