Hi there...I'm a regular spotter. I have a K20D with a Pentax FA 80-320, old but still reliable. I also use a Sigma 70-300 (a friend was not using it so she gave it to me). Most of my shooting happens during daylight (morning light is very special for this!) so you don't really need and ultra fast prime lens!. I would recommend a zoom lens up to 300mm, maybe the 55-300, or something like a 100-xxx. Beyond 300 would be a great addition but sometimes it's costly.
You can check my pics at:
Photos at Jetphotos.net
also at:
Photos at Airliners.net
I also have a website but my gallery is offline because I'm in the process of upgrading it. Here is my page:
CostaRicaAviation
Hope this helps!
Andre