Need input about best Diagnostic software purchase to Make

[bcredliner]

My take from several posts is that the GT1 is the way to go to have capability to diagnose any issue the computer knows about and to monitor the engine in real time? And that it works with Window XP? If that is the way I should go, can anyone list a link to the correct software to purchase. I have Googled GT1 and checked on Ebay and there seems to be a wide range of prices and items included that does nothing more than confuse my pea brain. I dug out a HP laptop that I used before going all Apple to use exclusively for this purpose (if I don't get a rash) that has Windows XP.