rebuild

im new to this site and am here seeking help or advice right now im in the process of rebuilding the engine and wanted to know what parts or kits were cheap but effective for the engine rebuild

I am thinking about buying a 2001 Mustang GT for about $4200, it has 125000 miles with quite a few upgrades (CAI, with tune 75mm throttle body, magnaflow exhaust, new brakes and rotors) all within the last 3000 miles I am thinking about rebuilding the engine, (will cost about $1000 I estimate)...