I showed this to dh who is a registered dental assistant (6.5 years of experience.) This is his response (Please don't sue us. ;)):
I'm used to looking at x-rays and photos as a basis for advice but I don't have that luxury here, so I will try to give you some advice so you can make an informed decision on your own.
The broken crown is probably something that can be ignored for awhile, assuming more porcelain doesn't chip off. If it does, as long as it doesn't open up a space between the teeth where food can get trapped, It's (relatively) stable. That's probably one of the last things to fix, but should be done at some point.
The fillings around the front teeth are probably discoloring because of leakage around the edge, which is to be expected with most composite fillings(especially ones that old.) If they have been in for 18 years, and there are no fractures, keep them clean and you could probably get more life out of them.
As for the molars, the old dentist was probably trying to do you a favor by placing a filling instead of a crown. It sounds like (from what you said he told you) he was not 100% sure if a filling would hold up long term, but he was confident it would last a few years at least. I know from working with many doctors that with an ambiguous diagnosis like that, they would probably be pleased that it held up as long as it has. Five years can be a long time in dental land depending on the situation, especially since a lot of them will only guarantee their work for a year or so(If you stop being their patient that is.)
As a general rule, what they teach in dental school is that if fillings take up more than 1/3 of the tooth, it should be a crown. Silver(amalgam) fillings that are that big can cause fractures over time that could become a big problem. Large composite fillings can leak over time and don't provide a great deal of support to the tooth. I have seen way too many of these guys break off, some end up needing root canals, some don't, but it's still a gamble. Molars take a lot of force and fillings are meant to fill a hole, not hold teeth together. Honestly, $350 isn't that expensive in terms of what a lot of doctors charge, but a root canal will probably double or triple that easily. If I was looking a a picture of it right now, and there were no fractures and no discoloration around the edge of the fillings, I would feel better about risking it. Just keep in mind that multiple fillings on multiple surfaces WILL eventually need to be taken care of, and a lack of discomfort is not a reliable indicator that there isn't a problem.
Hope this helps, good luck.