Speed and plastic is a bit of a double-edge sword (bit?) Depending on the thickness of the plastic, high speed can melt it. At the same time though, low speed may cause the bit or cutting wheel to clog up and jam halfway through the process.
One thing I would suggest is getting two Dremels - one AC powered one (the flex shaft is available separately also) for "big" work and one of the battery powered handheld ones for finer work. The battery ones run generally slower even at their highest speed than the AC units.