Well first of all, I'm sorry we all missed your post until now. Secondly, I can understand your Dad'* trepidation regarding advice from a website, as there are plenty of sites that spout out info without backing knowledge or that do not care for the welfare of the members. THIS IS NOT THAT KIND OF SITE!
As you probably already know, the owner and the admins here are very serious about providing valid advice based on experience or factual information that can be substantiated.
As for the light mods you are planning so far, there should not be any warranty worries. So far, you are on the right track. Most importantly, you are smart enough to do your research up front and and that is a big part of the battle.
IMO, if you do regular oil changes, check your fluids regularly, change your plugs and wires when it'* time and pay attention to what your car is telling you (I.E. listen to it run, note hesitations or misses, strange noises or feel anythin odd), you are already doing what is necessary to keep your car running for a long time.
I am personally not a proponent of changing the UIM/LIM gaskets just "because". Your car will almost always give you some indication that something is going wrong with enough warning to allow you a repair before real problems can occur.
Mike: 1997 SE w/ 183K, aka White Horse
, Gutted Airbox w/K&N, drilled 180* t-stat, tint all around, clear corners w/SilverStars, "PONTIAC" light up tail, PEMs, NGK TR55s