Need advice re long telephoto lenses

Hi,

I would like to get some advice from the long tele enthusiasts please........  I'm planning to try some bird and wildlife shots to add to my nature photography interests.  (Most of my experience is landscape, travel and macro).  I've nothing longer than a 100mm macro at the moment.  I've previously owned 50-150s and 70-200s and know they're nowhere near long enough.

I've done some research of the lenses available.  I can't afford super teles at all and I hate the mirror-tele look.

• The Pentax 55-300 is the cheap option, but 300mm is its poorest performing FL. 
• I might be able to afford a used FA or DA 300mm at around $800-1100.  They seem to have excellent reports. They're only 300mm, but are AF, and both have excellent IQ.
  
• The older MF Pentax M and A 400mm's come up reasonably regularly in ebay, where a good A400 might be $600. MF is a bit slow.  Although they're sharp, PF is reported to be an issue. 
• The Sigma EX 100-300/4 appears to be excellent, but that's still only 300mm.  They're discontinued and used ones are very rare.
• The last Sigma APO 400/5.6 is reported to be outstanding, but they're also discontinued and again, used ones are very rare. 

What would you recommend to a photographer needing a long tele from scratch?  Buy the DA or FA300?  And then either crop or use teleconverters?  Or buy an old Pentax A 400mm for the extra reach?  Is that extra reach worth the loss of AF and any more PF correction needed?

What are the better third party teleconverters for AF? And for MF - I'm guessing Pentax's A series converters are as good as any?   And do any of you know if the Pentax AF 1.7 teleconverter will work effectively on long f4 - 5.6 lenses?

Any comments appreciated.

Thanks, Rod