Deep Purple was also a "late" discover in my time, meaning that I started with iron Maiden, Black Sabbath, Judas Priest, Def Leppard, Saxon, Rush before getting to Deep Purple and Rainbow.
With Deep Purple you have to differentiate three eras basically Mk.I before 1971, Mk.II and III material up to 1975, 80's-today material. Being the classics the material from the Mk.II line-up (Gillan/Glover/Lord/Blackmore/Paice) most people consider it the material that has to be owned. I have only compilations and the live album "Made In Japan". Since I started with them at their return (1984) I have all their material since "Perfect Strangers" (one of their top albums ever).
Undoubtedly a band that has to be checked seriously to find a lot of basis for today bmaterial of bands, especially those into hard rock and the guitar wiz bands.
Not very familiar with Trapeze and Wishbone Ash material (forerunners of the twin lead guitar attack) but I just got hard into Uriah Heep a band that I paid attention superficially in the last 20 years. A friend got me "Demons And Wizards" as a birthday present and sparkled the whole interst in them again, so I just got their remasters and I recommend them seriously to all fine collectors and prog oriented ones in particular. At least the albums between "Very 'Eavy, Very 'Umble" (debut) and "The Magician's Birthday" are must be material, considering the ones from "Sweet Freedom" to "High And Mighty" as potential purchases (the ones with the Byron/Box/Hensley triad).