If you are going that rouet I would most certainly be putting in a wide band O2 sensor and looking at displaying AFR over oil temp/pressure.
Good idea, but budget big for something accurate. And TBH the ME9 (standard ECU) has plenty of protection for odd AF ratios and if it does start to run dangerously lean it will probably have caused the damage well before you spot it on the gauge, but this is never really an issue with a TFSI on original ECU.
I would go for an oil temp gauge. 50-180 degrees should be fine.