Doing an Opti is a $400 job just in parts if you do it right. At least $260 for the Opti itself EBay - thepartsladi), $30 for the timing cover seal kit, $100 for a new water pump (it has to come off, so if its old, replace it) and $40 in antifreeze, plus $5 to make the tool to instal the oil seal out of a highlighter.
Start by doing diagnostics. Black fouled plugs are a sign of running rich. How old are your O2 sensors and is your check engine light on?
2005 Pontiac GTO