You could definitely be sued if anyone is injured for doing this, and the sort of waiver you describe would be of almost no help or value--such waivers only protect against foreseeable or perceivable risks, but if you make an error in design or building equipment, such an error may not be foreseeable or perceivable to a customer or user, and so the waiver would not help. Also, to the extent you more more than "merely" negligent or careless, but were instead "grossly" (or very) negligent in what you did, the waiver would not protect you--you cannot contract away your liabiltiy for gross negligence, only simple negligence. By all means, have them sign the contracts, but don't count on the contracts too much. For more protection, set up the business as a corporation or LLC, to protect personal assets from business debts or judgments and make sure you have adequate insurance.