Cadillac Repair: Repairs to my Cadillacc 2003 CTS, cadillac dealer, piston rings

Question
Never had any issues with my car. Now of course that the warranty has expired and I am at 106,000 miles I am having problems  I just want to know if the amount the Cadillac Dealer wanted to charge me for repairs is an "honest" amount because at first they quoted me $1800 then they came in at $900. I got really concerned, sounded like they were trying to "rip me off".  I have taken my car into this dealer since purchased and never had this problem before.

Needed the valve and rings replace because engine oil leaking onto the spark plugs. Plus replace all the spark plugs.  

I parked my car and have driven it only to the grocery store. Now the car will not start. I do have power and car motor and fan belt turn on but does not "kick" in.

Must I have a Cadillac company work on the car or can I take to regular mechanic?
Thank You

Answer
They first quoted $1800.00 Now they say $900.00. There is something definately WRONG with this. There is NO way they can replace the piston rings on all the cylinders for $900.00.

I would get several different prices from most any automotive repair facilities that are familiar with Cadillac engines.