Marker,
They'll supply you with a kit for any car they have drawings to or you can supply to them. The overlays are made by the "The Train Station" who supplies HO kits as their primary business and has a pretty large collection of drawings for most major roads. So, the chances of getting what you want it very good.
I had them make up a Union Pacific COLA train for me and just need to find the time to build it.......which will take some time. I don't think these could be considered "beginner" kits but the assembly looks pretty straight forward if a modeler takes his time.
If you want railroad specific cars in "S" scale with proper window and door locations there is no other game in town. The kits come with side and roof overlays plus other body details and includes underbody details from BTS and another MFG.
You can buy them built up and painted or just built up but the delay could be timely since they have been discovered. Bob Hogan, who is an "S" modeler of considerable talent, built up most if not all the cars you see on the website. I'm told the fluted side kits are harder to install correctly than the non-fluted kits.
Kits were $99.00 without AM body and $120.00 with body if you don't have one.
Hope this helps.
Butch