here's a quick lens buying guide. This is all from my opinions, preferences and personal experiences so people will disagree, but trust me - I've put a ton of time into considering what gear I buy.
First, if you are going to buy glass, fuck DX (crop sensor specific) glass. Lenses will last forever if you treat them well, and every DSLR will be full frame before long. Don't waste your time
Don't ever buy a new lens that you aren't going to keep. If you are going to upgrade, buy used. Brand new lenses will lose value quick, but then level out and really never drop past a certain amount. That's why you see the same lenses all the time on craigslist and ebay for the same price. Also, if you aren't going to keep a lens forever, don't worry if it's got a few scratches. Odds are you will never ever notice, and a single hairline, almost invisible scratch will save you 100$.
Look at older versions of lenses. I have a fisheye from the 1970s that is sharper than the current one. It's just a tad darker and it's manual focus (which sucks sometimes but I don't really care about. It cost me 300 dollars used. the newest version costs like 900. I think you can even get older AF versions for 600-700.
KEH.com can suck balls, but not if you talk to Dan Orchard. Don't order online. Call KEH, ask to talk to him, and have him complete your order by phone. He's the only legit one there. If you are smart about it KEH is illllll.
I read Ken Rockwell's reviews before buying any glass. He has GREAT comparisons. Lots of people don't like him, and I would always read more than one review, but his website is reallllly useful and has almost every nikkor lens you can possibly think of there.
Also - this is just my opinion - but FUCK (most) 3rd party lenses. There are some gems out there, but Sigma, Tamron, etc, can all suck it. They lose value very quickly, and they very very rarely ever have better results. They also are more likely to end up with problems like loose focus rings, dirt, scratching, etc. Nikkor or bust.
Lastly, I know I said a bunch about "If you want to upgrade later", but i gotta say, buy the lenses you want. if it's truly what you want, you won't be disappointed, it will be worth every penny.