Unless you piss off the monkeys from SG most of the attacks came from spoiled kids that wont have bigg effect on a fine ass dedicated server even so you can always chose a nice hosting company that offer a decent DDoS protection.
Then learn some basics of C++ , PY , PHP most of the things you wanna do can be made in different ways and you must be able to use the easy one for example dont go always for c++ and fuck up your game core dayly when you can make the things with a simple quest.
Read most of the official quests there are some leaked ones from 2014 or wtf so you can understand how it works same with the files from Source in order to find a way to modify it as you wish
Ask for help when you stuck, google, check the tutorials on the forums dedicated to metin2 or clips its faster to learn like that.
About source pick 1 and work with it until you make it as you like you wont find any free or buy that will serve you as good as the one you manage to build yourself.
The players will not apreciate the things you do at the level of the effort you put into and think if the ideea you have in mind will fit in the gameplay you wanna build and how the players react to it before you start it not to work for nothing