I'll second the suggestion of the under-rated FA100-300/4.7-5.8 zoom (NOT the other one). I overpaid at US$120, but I'm not bitter.
I also think you should give your T80-210/4 a shot. Yes, be sure to prefocus, and also try Catch-In-Focus. But one issue with manual zooms is SR (shake reduction), deciding what focal length to tell the SR robot. In good light, no problem. In not-so-good light... well, it's hard to beat a tripod. Or just brace yourself.
Your Kx gives you a lot of ISO leeway for shooting at good speeds with the slower lens. If you can handle lens changes, various old manual 300/5.6's are available cheap, like for well under US$50, and with no SR issues. On your budget, don't worry about a fast lens; focal length and SR are more important concerns IMHO. Good luck!