general purchasing advice really depends on what you want. Do you want the cel, douga, genga, or all three? do you care about backgrounds? Displaying them or just collecting/storage. preservation/maintaining. cel damaged/vinegar syndrome. etc
There's three steps to cel purchasing but i would argue just matching the cel/douga you intend to purchase to a specific episode cut is the most important. a lot of the other steps come from having the cel in hand already
picrel