i agree that 100 psi and over 100'' of lift will be optimum. I have seen some units that are closer to 500 psi . . and their price reflects it . . .
i plan on getting the duramaid unit, which has only 60 psi . . . but still far, far more than a little green pro heat unit . . . the 40 extra psi will give you more penatration power. good luck with your decision . .