Meopta Axomat is robust and inexpensive and parts are available - though not sure of US secondhand availability. The Durst M370 or Graduate, as Clive suggested, are quite smooth in operation or a Durst C35 (though I could never like mine when I was a teenager). A Leitz Valoy can be picked up cheaply and could do what you want: there are a fair number of older type enlargers from the 1950's and 60's that can be had for very little. US models might include the Omega C700 and similar, which is light weight and probably more available.
Happy hunting, Mark Walker.