Agree with 4thStroke.
The most important thing with these car is maintenance. they are 17-18 years old and ususally are well into the 150-200k mile range. With a car this old, It is wise to figure for every dollar you spend in modifications to make it faster, plan to spend another dollar in general maintenance parts.
Obviously you will need to fully assess the condition of the car before going any further, but it seems that when you touch an "area" of the car it will always be a mix of upgrading one part and replacing worn parts around it at the same time.
IE...upgrade turbo usually involves fixing broken bolts, new gaskets, new oil/water lines.
upgrading the fuel pump often involves fixing broken sending unit/tank
upgrading the clutch: maintenance worn hydraulics & fork, CV boots, tie rod ends.
upgrade the cams: maintenance the Tbelt/water pump/Balance shaft area
typicaly you will find nearly every area of the car could use maintenance of some sort unless the car is somehow a low mileage cherry perfect example. Even then...parts age.
But to answer the question: the stock 6bolt 4g63 engine can be pushed past 600whp and live to see another day. There are a few experts who are going 140+mph trap speeds and low 10's on original stock shortblocks. My car has an original oem stock shortblock, but not quite that fast yet.