wheel/tire specs help needed

[68c10owner]

Hi guys. Long time member but new to this body style. Picking up a 2000 GMC 1500 2wd extended cab that the previous owner lowered. Im guesing its a 4/6. It still has the stock wheels and tires. Im looking for low cost wheel upgrades. I noticed later model OEM wheels is a popular option and some look great but what specs do I need to look at? I know its a 6x5.5 bolt pattern but thats all I know. Looking at 20" wheels, how wide can I go? What backspacing? And what tire size will fit front and rear? Not looking to go much shorter than a 45 series sidewall. Thanks for the input