Pearson 303, P30, Flyer
First, let me state my bias. I own a 1989 Pearson 27. Just want you to know, OK? As for build quality, I wouldn''t say there is much difference in overall quality between a P-30 and the 303. Do note that the 30 is a lot older design and that a LOT more of them were made than the 303. I have known 3 people well who own or owned 303''s, and none ever complained about the quality. The fact that many P-30''s are 30 years old and still sailing should speak for them.
As to what to look for, go back to the Buying a Boat category and look about 10 topics down for one titled along the lines of "What to look for in a Pearson 30." Some good info there. But if you want more opinions, Sailnet has a separate P-30 email list, so ask away there.
If you want to know more about the 303, send me a message or an email and I can get you the email address for a friend of mine who runs a 303 email list on his own. He will either know, or can get you the info from the list in no time.