Hi Tezza !
Pretty scarce engine you gonna use there ! It is the two-stroker boxer from Villers, am I right ?? Is it from the 60´ies - 70´ies ;o) ??
I think I have seen this on on a craft before, but I do not exactly remenber the name (need to check this out..). In this craft two villiers were used to drive the propulsion fans (not the lifting fan..).
For propulsion & lift, I guess Pascal is pretty right there, 18 HP are not really a lot & I guess you will need to build the craft EXTRA LIGHT but still rigid enough to archieve a convenient cushion... Probably you should look for a construction with another engine for lift as well..
By the way: Where do you come from exactly, for Pascal told me that you are from down under ( I worked there for a half year.. ;o) ) ??
Kind regards et a bientot,