What could I use to join parts of diecast cars together for a custom? Would solder do a good enough job or would some form of a glue be best?

I have never attempted soldering die cast metal but CA or 2 part epoxy glue will work very well.

Cheers.
Joe

If the part is tiny you risk melting it with the soldering iron, and if big it will act like a heat-sink and you'll never get it hot enough. Best to use either superglue or 5-min epoxy. (Mechanical connections are great, if it is possible to drill out and fit screws, but probably not much use in a kit where the parts have very little "meat" at mating surfaces to hold a screw-hole...)