You should be able to cantilever with no problem unless you expect to pick up something heavy such as isotope containers
The 8020 build method might work for you. Their plastic bearings and aluminum rails would seem to be perfectly suited to your need to be sterilizable and seem to be plenty accurate for pick and place operations. You may have to swap out for stainless fastener hardware such as screws and nuts; I'm not sure if they offer ss hardware.
Myself, I'd box out the X axis (four equidistant rails at the corners of a square profile) and run a stainless ballscrew in the center of the profile.
You will probably need to provide for hermetic motor enclosures of some sterilazable material, and avenues fo recirculating cooling air to the enclosures.
Motor size is going to depend on accelleration rates and traverse speeds factored around the masses and moment arms. Some of the people here can probably give you ballpark values off the top of their heads.