Is angel investment just for pre-VC companies?

While working on one business idea, I stumbled upon a second which may be even better. It's in a market that grew a lot in the last 10 years and has become a cash cow for the companies involved. We'd create a product over about 9 months, which might cost about $600,000 to build and launch. My back-of-the-envelope calculations show about $4 million possible annual revenue, even in year 1.

Obviously I need to test these numbers a lot before they'd be credible to investors. (And figure out the rest of the strategy!) But in general, a lot of my business ideas fall into this category. My ideas usually have a relatively modest cost to build, become cash flow positive quickly, but aren't necessarily huge ideas that would have VC appeal.

Do typical "angel investors" only invest in pre-VC companies? Are there angels that are receptive to smaller companies projecting relatively quick cash returns?

If not angels, what type of investors should I be looking for?