If you are worried about selling your current stroller because the company is out of business, then I wouldn't... <--not sure if that was a concern of yours or not.
We have discussed the P3 in many threads. Basically, they are an awesome stroller, and you would be very pleased with it. I just sold my 2004 P3 to someone here on the board, and I am missing it already.
No, the stroller will not tip with a rider on the back. It is very well balanced, so it pushes wonderful whether there is a rider on the back or not. They are about $330 new, but you can find them new online cheaper with free shipping and no tax. Sometimes they may be the model from the year before, but as far as I know everything 2004 and newer is great. In 2004 they overhauled the stroller including adding a snack tray with cup holder for the child, and also enlarged the ride plate. Before 2004 is just fine to, but I don't have much knowledge with those previous years.
Try searching or posting a wanted ad on your local Craigslist.
Here are some threads you can read where we discuss this stroller. Please feel free to ask any questions you have about it. I also have a ton of pictures of mine, including a video of some of it's funtions.
http://www.car-seat.org/showthread.php?t=13868&highlight=perego
http://www.car-seat.org/showthread.php?t=11263&highlight=perego
And here is a whole bunch of threads that the P3 stroller is mentioned in:
http://www.car-seat.org/search.php?searchid=115146