Welcome look forward to see the build play out. Like JettaGLI said def pick your tuner first and speak with them. Also as much as we all don't see eye to eye on everything here we all get along and happy to help where we can.
For my shameless plug if you have any interest in a cts kit cheap let me know i have one i am going to take off my mk5, i had it for sale before with trade ect but going to to sell it way cheaper. Its a gt3071 version, would include everything you need.
What are your plans for the mk5?