Anyone ever made a laser plotter cutter?

I would like to discuss a project I'm planning on doing and there are things I'm not sure about.
I'll be glad to photodocument this when I start building.

My idea for a laser plotter is to buy a cheap vinyl plotter from ebay, maybe even used. I already have one for regular vinyl work and its performs great. The reason for using one is that I'm not that much of an engineer to build one like some of the projects I read here on the forums.
My main goal is cut styrofoam logos/lettering. Thus my main question is what laser wattage would you recommend? I'm thinking 500-1000mW, but that's just a guess. Whoever has any experience with cutting styrofoam, could you describe how it went? If I could also burn pictures onto (but not trough) leather or plexiglass etc., that would be a nice bonus.

Now, the plotter would have to be disected, removing the bottom from the top to accomodate the thickness of styro desks (I'm thinking up to 5 or 10cm). The main difference would be that instead of the whole top of the plotter moving to cut, it would stay the same as with the vinyl foils, using the bottom rollers to move a platen of some sort, on which the styro desks would be layed (and fixed with something).
The vinyl plotters have a "knife-up, knife-down" mechanism (electromagnet?) for traveling to/from cut lines. I would use this for turning the laser on/off.

My only concerns are the speed at which the styro can be cut, becuase the plotters move pretty fast even set at lowest speed, though this could probably be further tuned, and keeping the letters in place, so they don't move since the lower roller of the plotters moves quickly. But if the cut was very thin, this may not be a problem.

Thanks for your thoughts!