Welcome!
Your standard unit is likely an RCD300 - not easy or econmical to fit bluetooth. Much better off going for a later VW unit e.g. RCD330+ (go for revE firmware) or RNS315 are popular choices. Both of these have bluetooth built in to the unit.
RCD 330+ VW unit for asian market where some newer cars are still built on the mk5 platform:
-bluetooth
-aux
-usb
-apple car play
-SD card (i think?)
-built in mic for phone calls (works surprisingly well)
-doesn't play so nicely with highline clocks or steering wheel buttons (i dont have these anyway)
RNS315 - EU VW unit:
-Bluetooth
-Sd
-Aux
-built in NAV
-DAB radio
-need to wire in a mic for phone calls
-play nicely with highline clocks/sterring wheel buttons (i think, dont have them)
other units are avaliable
I had the 315 and moved on to the RCD330+. Never looked back but both are good units. A google will bring up fitting guides and things to watch out for (clue: canbus gateway). They integrate nicely with the car, changing brightness in the dark, altering sounds level with speed etc etc.
As an aside - i thin 99% of components on the mk5 gti start "1k..." as 1k is the chassis type for the mk 5 platform