This can not be overstated. I think I bought my used kegs before I got my starter kit when I got into the hobby. There's a sticky in the kegging section that lists the McMaster part numbers for the Orings. I bought a 100 of the dip tube and post Orings and a 10 pack of the lid Orings. The last three used kegs I recently got from Adventures, I went ahead and added the Oring kit as I was needing a few more lid Orings. The kits had the white Orings and I think they are thicker than the black ones.
I have, currently, 12 used kegs. What I got in a habit of doing is weigh them empty and full. I take some blue painters tape and mark those values on them and even number them (ex: Keg 1, 2, 3, etc..) so it's better to keep track of them whenever an issue arises. I also use shipping tags tied to the lid that tell me what kind of beer is in the keg and I write on the back of the tag a reminder to double check any leak issues if I have any.
I like filling by weight, especially after getting my conical. I just fill the keg until I hot the target weight I wrote on the painters tape. This allow me to get the maximum amount of beer in them.