FSWizard(
https://app.fswizard.com/) says that's a 300N load, definitely achievable, stiffness wise.
one good way would be to weld the machine and then stress relief it at a local company. Whether your budget allows for it is up to you to find out. since you already have a machine, machining the surfaces could be done by you.
another way would be to use a ready-mix uhpc or mineral cast, one is a special concrete, the other is an epoxy based mix used for machine tools. an alternative to this would be a class c non shrink grout that's double compensated. this one is more of an experimental approach we're trying out in this forum, benefit is it's cheap but the results can vary from great to disastrous.
a lot of dental companies build their machines on and from a granite surface plate. that's also a possibility. this requires the use of a high speed spindle and a different machining approach. there are spindles that directly grab onto the endmill and they are available for 6.35mm endmills. the price for such a spindle would be high even from china, starting at about $2000.
material prices start at $3/kg for uhpc, $5/kg for mineral cast, $1.5/kg for grout.
If I were you, I'd start by digitizing my machine in cad and running fem on it, to be sure where you stand in terms of stiffness, then design something at least 2x or 3x stiffer. think it's a good place to start. you only need to translate the column and the spindle headstock with the spindle. grabcad might even have a model available for that purpose.