Did you consider to use a steel 26" wheeler which can be very sturdy, bear the heaviest loads and can be cheap - and mount S&S couplings? Really, if you don't need to fold for busses and trains, a separable bike could be more what you need for convenience. And this would be more likely to be doable in your budget. Did you search in the utility forum for bikes for heavy loads?
Or perhaps you can get a steel swift? Still, if I were you, I would try to improve the swift and smooth the ride by giving it the suspension by the already named means.