Gabroo,apologies for not having read your profile before posting,but you are obviously aware of all the problems.I have just looked at the Kaman K-Max site,and projecting the perf. graphs indicates some payload capability.However,manufacturers brochures should only be used as padding under helicopter seats;I would suggest however ,that there would be interest in approaching the Company,Kaman to see if they have any further performance data,or would be interested in your proposals/ideas.I only use the K-Max as an example,as it is very specialised,single pilot,big engine,no tail-rotor problems at lo-speed,no hydraulics,rugged construction;I`ve not flown one,but you may try `Rotorheads` to see if anyone has differing opinions/experience.
Alternatively,you could find an old Lama frame,get a g/box and rotor from a 355/365,find a big engine,split tail-rotor like an Apache,and Go-Fly !!!!