Good luck on the roller painting. Personally, it's too much work and there are ways you can achieve the same results with a lot less labor.
I think some people will be amazed at what a $60 Harbor Freight spray gun can do. As for the paint, Dupli-Color has come out with "Paint Shop" quarts of auto paint that is reasonable in price. Dupont also has a lower line of paint called Nason but I wouldn't recommend it based on my experience. As people say, it's mostly in the prep and post finishing.
I've painted 3 cars before and learned a lot from each one. I just can't imagine putting in 5+ x the amount of labor for a roller paint job for a < $5k car imho.
I know everyone's budget and living conditons are different but I would rather spray it or take it to a suitable Maaco than to do a roller paint job. Also, considering this is a BMW, you'll always encounter those elitest snobs although I tend to agree when it comes to roller painting a BMW.
Also unless you have clear coat, the paint will fade over time. I did a 1 stage enamel job before and it looked really good for 3 months after wetsanding and buffing. It was too much maintenance to keep it looking good thereafter.
I'm just trying to save you some headaches but you can obviously do as you wish.
My $.02