What factors are important when selecting a rotary tool?
To cut 0.4mm acrylic board I recommend a utility knife. What you do is draw a line for the cut, rest a straight edge on the line, then "draw" a line down the straight edge with the utility knife while putting only slight pressure down. You repeat this a few times until you have created a groove in the acrylic. Then you can begin pressing the knife down harder, and repeating until you cut through it.

To cut PVC pipe you can buy a cheap tool at Home Depot, Lowes, etc. that is quick and does an excellent job. The Kobalt 1/8-in to 1-1/8-in Copper Tube Cutter is an example.

As for a rotary tool, in my experience they are not very useful. The bits typically break easily, and dull quickly. The go-to tool for odd cuts for me is the Sonicrafter. There are a lot of other oscillating tools too. This tool makes cuts you just cannot make with any other tool.