A1 : there is a lot of serverfiles with a lot of systems like the systems you mentioned above ( pet , belt , acce (shoulder sash) , dungeons and some creepy systems
A2: you can implement or delete any system or make changes on systems but of course you need to know some programming languages for it like C++ , Python , Lua
adding or removing any system isn't big problem since there is a lot of systems included how to ( tutorials )
but I recommended to start with a clean serverfiles and you can add or remove anything
A3: the language of the files doesn't matter , it's possible to translate to any language you want if it was english or any other language you can change it easily
since you know C you can learn C++ and other language it will be more easy to understand other languages .
if you do any change in the source (server side or client side ) you need to compile it .
and yes there is tutorials to compile the binary or server side
sorry for my bad english