The BEST way is to include a provision for repair in the purchase contract and make the seller responsible. If you plan to take this on, I recommend you contact a professional that has experience in making repairs to acrylic and fiberglass tubs and shower pans. The final alternative is to do the repair yourself. The shower pan can be repaired using conventional fiberglass (reinforced plastic composite) repair methods. You will need a grinder, acetone solvent, fiberglass reinforcement cloth, 2-part polyester resin, polyester or epoxy filler putty, color matched gelcoat or and acrylic or lacquer paints. Many of these materials are available at plumbing or marine supply stores.
The methods are somewhat involved, and I will wait for you to respond that you intend to do a repair, before I detail them. Also, unless you have experience doing automotive or marine body work, do not expect an invisible repair. You are better off getting professional assistance.