You should only do a PPI through an authorised service centre. To provide you with added "peace of mind" you should also seek an experienced person to oversee that PPI. Now a lot of that insecurity from your side can be eliminated once you have sourced both an independent facility and person to oversee the job. That again can only be done when you know where the aircraft has been maintained throughout its previous life.
Feel free to PM me if you need anything further. Be careful out there, its a big investment you're making.