You can certainly understand the characterization of Roland just by reading the Gunslinger, but to answer these questions: "Does he want vengeance? Or just to find the dark tower? Does it change, is their revenge, or is it just a quest for the unknown?" You have to read the set. I would say you could skip one or two of them, namely Susana's Song and Wizard and Glass, but even those had some special information you wouldn't get at the end if you didn't read them. You can, however, skip entire pages.
PS: You don't have to read all of his books, but you definitely have to read Salems' Lot, The Stand, Eyes of the Dragon, It, Insomnia, The Talisman, The Shining (no, watching the movie doesn't count!).
And as for not being able to read his books because he puts too many out, I have read all of them. Except for The Stand because the first part bored me to death. But I did watch the movie. LOL.