Without an IR a PPL is restricted to a flight vis. of 3km, (without passengers) so could not take off in less.. .With an IMC rating minimum vis. for takeoff is 1800m. I don't think cloud base is specified for t/o, just the requirement to get down safely and legally, which also requires 1800m vis.and appropriate cloud base for the procedure.
I would be interested to know what a CPL holder has as take off limits with and without an IR.