Something ive discovered also. The Easiest way being a nice big long lens, I have a 70-200 2.8 and find at some points of tracks ive visted its fine, some require cropping. As for now i use a tele convertor to get more range from my gear i do want to upgrade but dont own a bottomless pit either, should the need arise or came into a cash injection im sure i could make it..
100-400L is fantastic lens ,i dont own one out of my budget for now, tho i do here good about he 150 -500 too and no TC is required at all, besides you would on manual focus if you did.
i agree IS is no use at motorsport, it fdoesnt work well for moving subjects or panning.
i went for the 70-200 1st knowing is was a good lens and i get some good use out of it, they are fairly prices 2nd hand also and can have a variety of uses
hope some of that waffle helps
best thing is to go for what you can afford with less range you will have to work harder, if it helps i have