The hassle as far as I can see about buying someone else's Subaru conversion is reverse engineering the conversion any time you come across a problem. In a well-engineered conversion EVERYTHING needs to be modified: engine mounting, gearbox, drive shafts, brakes, steering, ALL the electrics, plumbing has to be built from scratch etc. you'd need a dossier of the origin of all the parts in case they need replacements.
Unless you build it yourself it'll be a lot of hassle. Actually, to build it yourself will also be a lot of hassle.