I was in the same situation as you are. After i read a few threads on popular sizes, I just decided to start out with 12 designs (i have 80+ ideas), 3M, 5L, 3XL of each, because that's all i can afford right now.
I bought all my shirts at
Wholesale Blank Shirts - JiffyShirts.com and they are super fast with shipping. Got my clear plastic bags from
Boxes, Shipping Boxes, Shipping Supplies, Packaging Materials, Cardboard Boxes - ULINE, and now i'm almost done printing my designs.
Are you screen printing or heat pressing? If heat pressing, you'll be able to print to order (you can get shirts in 3 days or less), if not, i wouldn't advise you to take that route. The setup and cleanup wouldn't make you profitable unless, like you said, you extend the shipping time and let buyers aware. I know that's how T-Shirt Hell does it since they only offer black shirts for fast shipping. Everything else will come in 2 weeks. That is enough time to take orders, group them, print them, then ship out by the end of the second week.