What to look for in old wooden bodies planes?

I have been frequenting antique/junk malls lately, You know the kind where each merchant gets their little corner they rent out. Invariably some merchant has big glass case full of overpriced Kodak Brownies and wood planes. Usually a big jointer plane for upwards of $150, a couple of weird molding planes a few unmatched hollows and rounds. All for way more than I’d ever consider paying, BUT, I find if I wander back into the dimly lit ‘low rent district’ and brush away some cobwebs, and look for the corner with the pink doilies a porcelain doll, a tiny china tea set, and every shape, size, and, color of glass trinket imaginable, then look DOWN, I find all kinds a low priced goodies. Socket chisels marked ‘Sharp Screwdriver $1.50, or scratch awls marked ‘short ice pick 50 cents’ etc.

Anyway, in my travels I have spotted several wooden bodied planes, a few coffin planes, a rabbet plane or two and quite a few trying planes. All with price tags under $25. My question is what should I look for in buying an old used wooden bodies plane? What makes a good user plane? a good project? What is junk suited for some old ladies fireplace mantle. Beyond missing pieces I’m pretty clueless. Any tips, hints, suggestions or insight is appreciated.

-- Ted