INMO, 555 timers are unreliable. I'd have to locate a kit for a timing module that works off of 12 V. There is a timer that I'm thinking about that might cost about $30 and has a relay output. Otherwise, look at timing relays from
www.ssac.com. I'd say a ball-park price might be about $70.
If you don'e need much in terms of variable speed an AC synchronous motor could be used. It's not that hard to design a 12 VDC circuit to drive it at a variable speed. I've done it for a telescope for an acquaintance. He built it from my schematic.
In a final version this could be an embedded micro project. Just a thought.