I guess if I were just starting out, I wouldn't worry about having inventory on hand, just some demo lights. You can get product in 2-3 days.
But, if you are worried, my starter package would be:
-A few hundred warm white LED C9's
-A 1000' spool of spt-1 C9 wire with 12" spacing
- plugs for spt-1 wire
-a couple cases of warm white 5mm 70L strands
This will be a simple start for you that most customers are going to want anyway. Start simple and build off of that, or you will end up pulling in too many directions. Stick with C9's for roof lines so everything is standardized in your shop. Same for 5mm style lights.
Being that you are new, I would build around using LED"s. We are just a few years away from not having incandescent bulbs, so you don't want to get stuck trying to service lights you can't get anymore. Plus you'll have less failures, less extension cords, less power requirements. All which will save you time and money.