I echo the comment already made about the cost of construction products, I'd probably wait though I understand you wanting to have all your cars under the same roof.
I'll be 55 in May, I hope to move to Western MI in 3 years when our kids will all be in college, and then I hope/plan to big enough piece of land to build a shop that can hold 4-6 cars. Only reason I have 4 is I park 2 on the street though it's a very nice street.
When I was in my late 20s I had 5 cars...sigh.