I have a DS Cole kit... (which I have not had time to really get started on yet...).
I like the Cole for a casting kit because of its simplicity (very few parts).
I'd highly recommend it for a casting kit for that reason (could probably even be finished without a lathe and mill, except for the barrel threads).
If I was tooling up to sell these kits, I'd do two things to each casting before shipping it otherwise unfinished;
1- True and thread the barrel hole to give a rifle builder a single reference plane to work from, and since that's the hardest part for most home-shop setups to do... internal threads on a hard to fixture and hold part.
2- Centerpunch (probably using a fixture) where holes go. The "cast in dimples" are really not accurate for hole location (at least on mine). Punch-marks in accurate locations would help enormously.
Of course, if I ever actually tackle and FINISH the DS Cole action, I'll know if it's any darn good
Aside from that, I think your friend is thinking right... Breechblock blanks for High Walls, and Ballards, and Lever castings for High Walls would be awfully nice to be able to get. So would some buttplate castings.
Just my opinions, of course...
Paul F.