1994 z28 camaro transmission/shift problems

So my buddy has a 1994 z28, auto, that i'm going to buy if we can get it fixed. It only has 45,000 miles on it and it has rims, exhaust, racing chip, cold air intake.. a list of small mods. He didnt drive
it in the winter because we live in indiana but before winter he had the transmission worked on. They put on a shift kit and a slip kit..? i'm not exactly sure what that means all i know is now the transmission is messed up. It does not shift properly when you:
accelerate very slowly it will go up to over 4000 rpms and then finally shift, but if you back off the gas and slowly pick it back up it will shift
when you completely mash on it 0mph to second gear it doesnt shift when it needs to. it will tach out a couple of times an then when it does shift and vibrates very bad, if you let off the gas to shift as if it was a manual when you get back into the gas it should shift correct? wrong it tachs out a couple times and then shifts hard into 2nd
if you put the car into 2nd and not drive it wont shift it will accelerate and then tach out until you back off
also when you have the cruise set at 55-60 mph it runs in third gear not 4th.. thus making the rpm's run high at 2500rpm
the transmission fluid is ungodly dirty and nearly looks like oil even though it is supposed to be red... if we flush the transmission will it mess up the transmission or will it even fix the shift problem?
could the shift kit and slip kit possibly been done wrong? if so wouldn't that of been noticed earlier? or could it be since it's sat for a few months and with the bad trans fluid be making the car not shift properly? or expensive as it would be... is the transmission going bad due to hard driving?
my friend has owned the car for almost 2 summers and only driven it for maybe 8,000 miles... he drives it hard but would just normal hard driving cause the tranny to go bad at only 45,000 miles? i know this is long and probably very confusing but hopefully someone will take the time to help me out! thanks!

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