how long will the v6 in the ml 320 run with the proper attention?

Depends on where you decide the cost of keeping the engine running is more than the car is worth to you. Generally, you can expect to get about 300,000-500,000 miles out of a care if you perform all scheduled maintenance at proper intervals and make sure that any work you don't do yourself is done by a very good mechanic.

1999 ml320 i'm looking to buy has 227000 on it is that good or bad ?

My 2002 ML320 has now accumulated 325,556 miles and this vehicle is a one-owner car. I bought it in April of 2002 and I still have it. The only issue i have is the air conditioning not performing when I'm in a Stop N' Go situation. But on long drives the aircon is superb. . . What should be replaced on my aircon system??

Freon, possibly ac clutch if it has one