As per Lincoln Electric information on welding 4130...

Q. Do I need to pre-heat?
A. Thin wall tubing (< 0.120" wall) applications do not typically require the normal 300ºF to 400ºF pre-heat to obtain acceptable results. However, tubing should be at room temperature (70ºF) or above before welding.

1020 MS is not bad. 1010, typically used in welded seam stuff is soft. I would not consider any ERW tubing. Most of the cheap kits use this. Personally, I would not use anything but 4130, unless it positively had to be MIGed. It's just better. Weld it, drill it, machine it, bend it. The difference is noticeable and substantial. The cost difference between CM and MS is negligible when talking about DOM. If the car will be TIGed, it's a no brainer.