If I'm counting correctly, the 700 series of T&S has 2 sets of plastic gears. It has a rubber top on the feed dogs, which deteriorates with time, but it's an easy fix and not that expensive. Without a doubt, it is not as good as a 401 or a 501. However, it does a lot of things that the older slant machines won't do. I like the machines, as I have one I bought new. It hasn't seen much use, though. I love the bobbin filling thingy. I have three 750s that still work.
The replacement gears aren't that expensive if you can/will do the replacement yourself. It's only worth the time and money if you want to use the machine yourself. You certainly cannot recoup your investment in time and money when/if you try to sell them. I've heard the figure of $160.00 to have them replaced.
bkay