don't worry about the $400 machines being plastic inside, the $4000 machine are too.... if you want strong, stick with vintage, Pfaff, Viking, Bernina, Singers (but even older) Husqvarna, there are many older machines out there that were not used a lot and still have a lot of life in them.... pick up the machine, how heavy is it?...it is a good indicator of how much metal is inside, sew on it at the store and have thin, medium and heavy weight fabrics with you... do some stitches that zigzag, lots of straight stitches, make a buttonhole, .... how is the noise level? you want it fairly quiet but also how smooth-running does it sound? .... does it vibrate when running at high speed? now really look at that buttonhole....is it smooth on top? will it look better if you adjust the tension? most buttonholes and decorative stitches need the tension to go down one number to look really nice .... i have bought several machines in my lifetime and if they won't let you sew on the machine, you don't want it.... how many feet come with the machine? if you are happy with the way it sews and sounds then ask about any guarantee they offer for their used machines... and while buying, ask about any extra feet that come with it or that might be available... you will get the best deal on them at the time of purchase...