1. It can be found on your clientinfo inside your client folder or ragnarok folder. Change the admin whatsoever that is or the ID number and it'll remove the GM sprites. (Please search the search box, there are tons of help. -- Do not be lazy.)
2. You can change the drop rates at ( rathena/conf/battle/drops.conf ) -- I haven't tried that guide so maybe there's no conf, idk.
You can change the exp rates at ( rathena/conf/battle/exp.conf ) -- Again, look above. 3. Didn't tackle this yet, so I don't know, but if I do, you have to tackle some mob.db or whatever.
4. To add a customized item, you have to make some editing on your item db and the client folder. Do not be lazy, search it on the search box.
5. You can find this at rathena/npc/mapflag or again, I haven't tried that guide and don't know if it's updated so maybe there's no mapflag something. and just follow whatever it stated there.