The weather-stripping we use on our PVC door frame, is the same as you would buy for a wood framed door at any home hardware type stores. It is also replaced in the same manner, though our's may hook into the frame more positively.

Use a pair of pliers to grab the end of the weather-strip near the end, then pull it out of the kerf in the frame. You will have to do this on each side and the top.

Cut the new weather-stripping to length, and mitre cut the top corners so that they overlap somewhat. Use the old weather-strip pieces as a reference.

You install the new weather-stripping by simply pushing it into the kerf in the PVC door frame. You can use a flat piece of wood approximately 50mm (2") wide with a blunt end to push it in if your fingers alone aren't enough pressure.

The door strikes we install on our doors when they leave the factory are adjustable. Loosen the two screws that hold it in place and adjust the two overlapping pieces of metal that make-up the strike.  Position the two overlapping parts in the position you think is adequate and tighten the two screws.  Close the door and test how tightly it closes, repeat till the door panel locks in the position you need to get an air tight seal.