300E Sucking Oil

We've noticed over the last four months that we've had to add a quart of oil every 600 miles to our 300E.

Our mechanic did a leak down test and says that in cylinder 1 to 3 we have 10% leakage and in cylinder 4 to 6 we have 70 to 80% leakage.

He also told us that our spark plugs were full of oil.

Our questions are these:

1.What's a fair price for fixing this problem?

2.Can we still drive the car safely by changing the plugs occasionally and adding more oil until we can afford to have it fixed in a couple of months?

3.Would this problem be better fixed by a dealership than an independant shop?

P.S. This car has 105,000 miles on it and runs great. Other than having the oil problem, we have no complaints.