I've only shot a couple of car shows, one was on a rainy day, and at that time my only WR lens was the DA 18-35. The other car show was indoors and I used the DA 15 only. and I had the I have since done an indoor motorcycle show where I used the DA 18-135, DA 15, Rokinon 8mm fisheye, and the Tamron 10-24.
DA 18-135
The 18-135 was nice to use for some closeups when I wanted detail. The first shot is what 90% of people take at car shows, pretty boring, OK for just seeing what the cars look like, but just a blah photo.
DA 15
The 15 was really nice because you can get close without a lot of distortion, plus that Limited IQ.
Tamron 10-24
The 10-24 offered the chance to get pretty close, but with distortion. I no longer have the 10-24, I really used it less and less after getting the 15. One problem with the 10-24 besides the distortion is flare, it doesn't have the Pentax coatings. I like to shoot at a low vantage point sometimes, and if it is bright and sunny, it's hard to keep the light off of that 77mm front element.
A lens I did not have at the time of these shows but do have now is the DA 20-40. It would definitely go with me now, if I only took one lens that would be the one, it just handles everything well. If I took two, I would probably take the 15 along, and that would probably be enough. Of the lenses you mention I would choose the DA20-40, it would cover you for low light and possible rain. If you take a second lens the Tamron is wide enough to get by with a slow shutter.
But I really think the composition is more important than the lens, trying to make the photos interesting is more fun. I would take a small tripod also, if it's not too much to carry.