What Primer?

The best bet is always to get the primer etcher/sealer of the paint system you are going to use on the bare metal. If you don't use a etcher/sealer then the primer will not adhere to the metal (etcher part) and without the sealer, moisture will permiate the rattle can primer and may start rusting out underneath.

If your budget is totally shot, and the doors are in bare metal, then any primer is better than no primer. You can get the rattle can primer from any Wal-Mart, just keep in mind it should be removed again right before you apply the primer etcher/sealer of your paint system. Ideally the car should be kept in the garage and not exposed to any moisture (rain or lawn overspray).

I've heard of some people using a product that you coat the bare steel that will not allow rust to gain a foothold, but I cna't remember what it was. You can also use oil, but it will cause havoc when you go to paint.

You can use pickle-x to stop rust till you can paint it. google it and you will find it

Most use a epoxy primer nowadays instead of etching primer because it seals and has a good build to it.

Just remember like JB1dsl stated, self-etching primer will just prepair the metal and adhere to the metal, but it won't seal the metal from moisture & rust, but it will hold it at bay for a while (especially if you're in a drier climate like AZ)