Hatching is addictive, and that is where you need to be careful. 100 x day old chicks are very cute and don't take up much space. Roll on 3 or 4 months and it's a different story.
If you want to hatch to sell, then start with good examples of a pure breed and go from there. Not many people want to buy cross breeds, and have a plan for all the males as I mentioned earlier.
There is no need to vaccinate your stock. As Chris says it's only really done in commercial hatcheries where they are producing 1000s of chicks.
I used to breed - mainly Welsummers, Marans and Sussex - but the one thing I found really quite stressful was the selling part. I hated the birds going off to a home that I knew nothing about after my months of hard work. You can't hand pick your buyers, at least not if you are selling in any quantity, and there are only so many friends and family to sell to!!