Speaking as a recovering Ad Industry professional:
Do NOT build an Ad Server unless you are focussing on the ad server business. There are way more industry particulars than you want to deal with.
Use an ad network to start with. Or a few of them. This will git you some fill and some revenue. Later you can use this for backfill.
You'll make more money ultimately by selling direct and by having something that appeals to major advertisers. But you need scale and you'll want an ad sales team.
When you do sell direct, pricing varies widely with a huge number of factors. The biggest are
* the value and size of your audience
* perceived value of your brand
* modality of your users
* types of "Ad Products" you are selling
* various available targeting parameters
If you are trying to figure out what it might look like for modeling purposes, I would suggest a little old-fashioned skullduggery. Find a comparable set of established publishers/apps. Then: find a friend at a media agency (or learn enough about the business to sound believable) and ask them to get some rate cards and quotes for you. Depending on the industry, you can often find the rate cards on their site -- but keep in mind these prices are negotiated .
CPM vs CPC really depends on the kind of ad/advertiser, and CPC usually backs out to an eCPM anyway. Premium brand advertising is typically CPM. Direct response (acquisition) is usually CPC. Everything else runs the gamut. I wouldn't worry about this too much at the moment. If you're using a network (networks), look at what's working best for your revenue and customer experience and optimize accordingly.
Is this mobile or desktop?