I ended up using what I had, the S5 with the 80-200 AF-S and a Tamron 1.4x for the "action". I was lucky, and had lots of light, so I could use ISO 400 and still get short shutter times and small enough apertures to get a reasonable DOF. I did buy a new tripod though, a light Benro, since I had to carry it around most of the day. I will need a better ballhead for panning. RRS has an excellent one with pan both at the base and at the head. Rather costly though, but for motorsports, it's probably worth it.
Mark Rebilas, who posts at the sportsforum at FM, has an excellent blog where he describes equipment and techniques. Very informative.
There are different target groups for these photos, so I need a wide variety. In addition, I take the photos that I like to take, like the VIP shot, that isn't really useful, but lots of fun to take
I will most probably buy one of the Sigma lenses (100-300 f/4 or, if I can afford it, 120-300 f/2.8. I do need more reach, and a TC isn't an ideal solution. The S5 focuses just fine, and high frame rates isn't that relevant with motorsports. Finding "The decisive moment" is much more important. What I will need sooner or later, is better high ISO for darker days. The D700 is an obvious candidate, but I lose one stop if I need to use a TC to maintain the same reach. Dilemmas, dilemmas...