my friend bought a 401 today for $75 in a cabinet, both in very good shape. i have a 503a Rocketeer that i absolutely love - it's been my only machine since 1989 (til now when i've fallen in love with all of these black beauties!) and i've never had a single problem with it. i oil and lube it and i have made tons of clothes and at least a dozen quilts on it. i love that machine.
however, missing a top . . . i'd make sure i could get a replacement part on ebay before i bought it - or i'd dicker her down to about $25. while it doesn't serve an official function, other than keeping dust out, to me, it's a significant part of the appearance. i have seen the tops on ebay, so look and you may get lucky.