My first experiences and only with plastic gears was with two Singer T&S model 628 and 648, late 60's vintage. I was pretty happy to get both for $10 and they both looked fine. I busted up a gear on each one as soon as each started running. Never fooled with plastic gears again. I figure there are too many good machines to be had cheaply and life is too short to fool with them. An exception to this is a machine with family history or for the experience of replacing gears.
On the flip side, there are Singers and some others with all metal gears but are poorly designed or have weak internal motors which I would avoid.
Jon