For Telephoto work - D7000, D5200, D300s, or wait ?

depends on your timing.

if you need to buy today:  used D300s + 70-200 + TC14

if you can wait, compare D5200 to what is released in first half next year.  i wouldn't want to accept the viewfinder compromises on the 5200.  and i expect a D400 class camera to be released that may exactly fit your need.

i recommend the first option because you can always dump the D300s for near the price you paid for it and you'll never regret the 70-200 purchase unless you spend most of your quality time >400mm

...dav