Get a Bentley' manual, replace all the plastic piping/sensors in the cooling system, flush and refill with the proper red VW coolant. Replace any hoses that look bad.
For an example of the plastic crap-
Get quality oem parts from places like germanautoparts.com, ecstuning.com, etc. not autozone junk.
Timing belt service should include complete replacement of all related parts- belt, water pump w/metal (not plastic) impeller, tensioner, thermostat, plastic thermostat housing, and new motor mount bolts.
Most german parts specialty places stock auto trans fluid/filter/gasket kits for about $100 or so. Do yours now. By 100k it's too late.