Ask HN: Validating an ad supported business?

I'm considering an idea for a content business which would be ad supported. There's an awful lot of advice out there on avoiding this kind of business in favour of something where people pay you directly, but let's say I want to consider an ad supported website as a business - how would I model / validate it?

My thinking so far is that it's a bit like the marketplace problem. On one side of the marketplace you have users of your site, and you can validate their interest by making some kind of MVP and looking at traction. On the other you have advertisers, and while you can estimate some kind of industry standard eCPM as a clearing price, the real money to be made with individual deals which aren't really on the table until you have a certain size of user base.

Anyone have pointers on this from past experience? Anyone think it should be avoided at all costs?