So I have never painted a play mobil arena but as an avid plastic model builder It is possible if you want it to look good and last there are some very simple steps to take and it will look amazing!
1. Most Important WASH EVERY PIECE YOU WANT TO PAINT IN HOT SOAPY WATER AND DO NOT TOUCH IT WITH YOUR BARE SKIN AGAIN! wear gloves! and make sure you dry it well.
2. Wipe the entire surface area down with rubbing alcohol and paper towels right before you paint each piece
3.make sure you paint is suitable for plastic I.E. Krylon Fusion,
http://www.krylon.com/products/fusion-for-plastic/ there are other paints out there this is just the first one I thought of
4. you want to prime it using a plastic suitable paint using many very thin layers do this by holding the can further away from the piece you are painting I'm talking at least 5 layers
5 using the thin layer technique lay down your textured color
6. using thin layers add 2-4 coats GLOSSY clear coat it adds protection!
7. using thin layers add your satin/matte/flat clear coat just enough to get all the shiny gone!
and remember at least 20 minutes between coats of paint and NEVER paint when the humidity is over 60% it will cause frosting!