Roof Estimate - Tear off old shingles?

[Boots Bonner]

I am replacing a 17 year old roof on a 2-story 1400 sq ft. zero lot line (half a duplex) in Murfreesboro, TN.  The roofer wants to tear off existing roof (only 1 layer of shingles) and replace with 3 tab 60 mph rated shingles. There is currently a leak around the vent pipes and those would be replaced. His bid is 4500 - for my half of the roof. It will be $9,000 if the neighbor replaces their half of the roof. In addition there would be a $500 charge to replace some damaged steel fascia.

Questions:

1. Do I need tear off the shingles or can we just add a layer of shingles?
2. Is the price in line with approximately 700 sq ft of roof?