Aftermarket HeadUnit Questions.

I am purchasing an aftermarket HeadUnit for my '02 Tahoe. I have a friend who has an Impala who said she couldn't get an aftermarket HU because the computer was located in the stock HU and something wouldn't work right if they replaced it. Am I looking at the same problem in my SUV? I was thinking about also getting a Viper Remote Start/Keyless Entry system as well. What should I watch out for? Anyone had problems with this? Thanks!