A *300 really is the place to go, as others have said. I've seen some lately going for around $800 on eBay - specifically the F*, which is my favorite anyway (and the one I have). But make sure it's a good copy, functions correctly, and doesn't have a problem such as fungus (this applies to any lens - in rare cases could even be possible with new ones, depending on how they've been shipped/stored). You need to learn how to check for fungus anyway if you don't know already, now that you're venturing into mid-range to higher-priced lenses. Many of the spare parts are no longer available from Pentax for the F*, so it's hard to get someone to repair it.
Don't be scared off, however. Many others on the forum have great copies of the F*300 as well, and it tends to be the favorite lens in their kit. But I think it's much wiser to buy used most of the time. If you're really interested in birding, then put all other photography-related purchases on hold and get one of the *300 lenses. You can resell it and get your money back if you don't like it, so the risk is minimal, unless you plan on severely damaging it
. An if you like it then you can add a 1.4x TC next.
Look at the images on the PPG for all three *300 AF lenses:
PENTAX : Select a PENTAX interchangeable lens camera or a lens model PENTAX : Select a PENTAX interchangeable lens camera or a lens model PENTAX : Select a PENTAX interchangeable lens camera or a lens model
They're all great lenses with excellent Image Quality. My evaluation of the (sometimes subtle) differences among the three is:
F* - smallest, lightest, handy built in hood, best IQ, tripod collar (which you may never end up using), limited spare parts (hard to get anyone to repair it because they want to be sure they can reassemble it if anything breaks in the process)
FA* - no tripod collar, kind of a compromise between the other two in almost every way (which isn't necessarily a bad thing)
DA* - "worst" IQ (but still excellent), heaviest, tripod collar, WR, somewhat slower AF, very quiet AF, 1/3 stop wider aperture (not very significant), available new or used.