Just to toss a little more info in here............
It might help to understand that when people talk about primers they're not always talking about the same thing. As HRP said, you'll get different answers from different folks because they bring different experiences to the table.

As an example from DW's (spraytech) list, there are two primer catagories; the DP which is an epoxy primer/sealer, and the acrylic urethanes K or NCP which are primer/surfacer (but can also be used as a wet on wet sealer with proper procedure). In simplified terms the epoxy primers adhere chemically, the acrylics mechanically (the roughened surface HRP noted). Many believe, and have sucessfully shown, that the chemical bond of the epoxy is "better" than the mechanical bond of the body filler on bare metal. Years of experience doing it the other way successfully would be an adequate argument against that. What the epoxy primer does do effectively though is act as another barrier against moisture migration as body filler is very porous, like a sponge. If you seal it off with primer quickly, that's not much of an issue, but if you let it sit, and there's no barrier between the bare metal and the body filler, then rust will occur under the filler. Atmospheric conditions will dictate how much, how quickly. If you've ever taken off old body work done over bare metal you've likely found surface rust.

As DW said, ppg will allow the use of body filler over the DP products.

As for your top coat, if you're going with ppg you could use DCC black with some flattener. You control the amount of flattener you add, thus the amount of sheen you get. Given the cost of clears a single stage like DCC would be more cost effective.