How much to replace a timing chain on a 2000 vw passat--1.8 turbo-- should i do this?--it has 113,000 miles on it and it runs great.-- have changed the oil on it every 5000 miles since new--

It's a turbo problem of sorts. Have a look on the following link which will give you access to more information:

passat eror code 17965 Google Search

To be quite honest, there's nobody on Fixya who will be able to give you a definitive answer. Turbos can suffer from a number of faults and those faults can sometimes be quite difficult to track down.

Here's a link to Ebay:
passat turbo

It may be worth your while actually contacting a turbo specialist and asking their opinion. What you don't want to do is fit part after part in hope of curing the problem. Your first course of action should be to ask a turbo specialist or good workshop. Yes, it may mean paying for somebody to check things out for you, but it really does save money and bother in the long run.

Between 105 K to 110 K stretching it 120 K

More detail would be nice, but I can give you a worst case scenario..
The mk4 Passat 1.8t has been plagued with clogged oil pumps. The oil change intervals might have been too infrequent, sludge builds up, and the only fix is to drop the oil pan and clean or replace the pump.
Just a wild guess but insufficient lubrication and low oil pressure/volume could have led to engine damage. Its hard to say without hearing it.
In the mean time check your maintenance on the car, if it has gone more than 60,000 miles without a new timing belt and tensioner, fix that right away!! When those belts give up the ghost, its time to invest in a new engine.

the timing belt or chain...which ever you have...
a transmission oil change...every 3,000 miles or 3 months you need an engine oil change...so check that...
the spark plugs and wires,change the rotor if it hasn't been changed for a while...and check if the radiator needs flushing...
Good Luck!

Belt drive turbo? loose belt....

yes !

mass meter is going out and the flashes mean the car is misfirng probably the coils

I had the same problem with my wife's 2006 passat turbo. We lost 2.5 quarts in 3000 miles and the warning light came on. They put us on a 1000 mile oil test and I lost 1.8 more quarts of oil. They finally agreed to replace the rings. Service tech told me that the factory rings are defective, they begin to lead after about 25,000.

The flashing is telling you that the engine is mis-firing, the dealer will run a test to determine the cylinder that is mis-firing, chances are it is something simple