Well, the MAF is a big standout right now as a problem; a bad maf, can make the car run horribly and cause rich or lean codes, etc.
Do you still have the original maf(?); it maybe fine. You might clean it snd reinstall; see if the poor running problem goes away.
If you end up needing a new maf; stick with Bosch oem only, you can look up the right one here:
Buy from a reputable company; as we seem to be seeing Bosch counterfeit parts from China from eBay and Amazon.
Lately, FCPEURO, has been a great VW parts supplier and they offer free shipping, lifetime warranty on everything they sell.
As to the scan tool; a vw specific one would be ideal (VCDS, OBDEleven, “VAG” scanner etc).
For now, we need to read some basic info; like trouble codes and live data, for example the maf readings, fuel trim, etc. Any auto parts stores; should be able to rent you a scanner they can read codes and view live data.
If you want to purchase a VW specific scanner; let us know your budget and what you want to have, etc. The investment in a factory VW scanner; will save you money, enable you to diagnose things and complete repairs. Most VW shops and VW dealerships; are over $100 per hour labor rate and so, it doesn’t take much, to pay for the cost of the scan tool, then you have it for future repairs.