I will suggest that you locate a copy of the May 1997 issue of Model Railroader Magazine., and probably an issue or two after that. In the May '97 issue is Part #1 of an article titled, "Railroads and the Automobile Age". The article is about 10 pages and provides just the suggestions you need for such a layout and how to model it in limited space as well as operating suggestions for just such a model railroad (inbound and outbound loads, etc). I assume there is a Part #2 in the next issue, but I don't have that issue.
Next, I would suggest locating a Walthers Catalog from 2000. That year, Walthers introduced a line of HO scale Structure Kits in a line they called "America's Driving Force". These structures represented auto industry structures. Some may still be available, others are discontinued, and some may have been repurposed into different kits. I believe the kits were based on actual structures of Ford Motor Company. The auto industry kits included:
#933 3074 Headquarters Building 100.00
#933 3075 Stamping Plant 70.00
#933 3077 Uptown Motors Auto Dealership 55.00
#933 3076 Distribution Facility. 100.00
#933 3078 Tire Plant 150.00
#933 3079 Auto Assembly Plant. 80.00
I suggest finding the catalog as it suggests rolling stock and other products to use with these structures. Some components of the old discontinued kits may show up as part of other structures in current kits.
Someone makes, or made, flat car loads of automobile frames. Can't remember who made that, but it might be something worth looking for that could be stacked around a structure or as a flatcar load. It might have been "Scenemaster".
http://www.bgtrainworld.com/storefront/shop/item.aspx?itemid=2
If you decide to expand the layout to include the steel industry supplying the auto industry, the 2000 Walthers Catalog also has a full line of (now mostly discontinued) structures for the steel industry that could give you some scratch building ideas if you can't find the original kits.
Good luck. I want to see pictures of your work.