Depends on how much comes with the engine.
The mk3 VR6 gearbox is different to GTIs.
The VR6 clutch is hydraulic rather than cable as found in GTIs.
I guess different manifolds.
VR6 dials would be a good idea, ie 160mph and correct rev counter - redline at just under 7k rpm.
Suspension upgrade is a must
Brake upgrade would be advisable unless you have very late spec gti brakes i.e 288mm front discs.
The VR6 has twin cooling fans and possibly a different radiator (same as GTI 16v though)
VR6 ECU
I'm not sure if you need all of this stuff but it's just some ideas.