Q:
+ What files should I have?
Client, GRF, related data files etc. For the server, that trunkOLD download has everything you'd need.
+ Where should they be located?
Anywhere you want.
+ Which client should be used, that doesn't contain sprite\shadow errors? -
The client doesn't matter for this, it depends on how updated your data.grf and rdata.grf are.
+ What folder should the client be in? (Should it be in the same folder with the batch files/runserver?)
It can go wherever, you don't keep it in the same folder as the server.
+ Should the data folder be inside the same folder with the batch files? (to avoid client errors?)
No? It has nothing to do with the server files.
+ Does it require a full kRO update?
Sorta, read my answer to the next question.
+ Does it work with kRO Renewal?
eAthena (especially an old version) will not have any renewal features. No 3rd jobs, no mounts, no Kagerou / Oboro, etc. No renewal mechanics either. Basically nothing after episode 13.3 I believe.