The PI heads are the factory heads on the 99+ alot of companies sell them, check Foxlkae racing, livernois motorsports, houston performance, ford modular powerhouse should get you in the right direction. But I'm sure you can shop around some more and get a really good deal. Yeah with 80K I wouldn't but if the motor has been well maintained is problem free you could but I think your taking a bigger risk. The heads gears and LT's and then get a good dyno tune and maybe some cams and your gonna be alot better off!
Eventually look at the tranny and get a torque converter that will wake your car up big time