Since you're 18, insurance will be high on this Sierra, since it's probably a Cat 18. So, expect to be paying some high sums.
I disagree about the 2.0 I4 being junk, but if you must do ONE modification, make sure you wire the
electric fans to be on all the time.
These engines will blow head gaskets if they overheat, and usually it's due to the fan switch giving up the ghost. You will thank me for it later. Also change the radiator fluid once a year, and change the timing chain & tensioner as soon as you buy it - then change that once every five years (you'd probably have flogged it by then).