Thanks for the kind words, I'll be continuing with the improvements as I get time.
I use the standard MTH steam smoke unit, AA-0000070. for many upgrades. There's also a diesel model, the only difference is it has longer mounting posts.
The weights I use are 1oz automotive self-adhesive wheel weights. I just stick them around where they fit, trying to keep them low and symmetrical from side to side and centered as much as possible. The extra 8 or 10 ounces doesn't affect the pulling power enough to talk about, but it does give the light tender a lot more stability. The locomotive didn't miss a beat pulling the four cars around the club layout, including grades. Since all I expect it'll usually be pulling is it's passenger cars, I don't think I have a traction issue.
The flashing car is indeed the Dementors Car, that's the one I want to do something fancy in with the lighting. I have the audio clips to add sound as well, and I'll be triggering them using TMCC. It might be cool to have normal lighting and then be able to go to the weird lighting on command.