Is $4,500 to repair and waterproof a leaking flat roof a fair price?

My townhouse roof is leaking, and it's probably 20-25 years old. The leaks seem to be coming from a vent and pipes from the AC system installed on the roof. The roofer also noted cracks on different areas of the membrane. I'm wondering if it’s enough to repair and reseal and waterproof or if I need to redo the whole roof. The roofer says I won’t have leaks for possible 5-10 years if he repairs, but that doesn’t come with a warranty. If he replaces the roof, the warranty is 10 years. I also wonder if the prices are right for an 18x40’ roof. I'm concerned about installing a new roof on top of the old one, especially if it will trap moisture. An architect friend mentioned adding insulation boards before a new roof, as the top floor of my building gets really hot in summer. I need to know if the attic area will eventually dry out even if I don’t tear off the old roof, and how long I should keep the drywalls open. I also thought insurance typically doesn’t cover the cost of replacing an old roof. I’m going to get 2 more quotes tomorrow and hopefully a 4th one by Tuesday, as I need to act quickly before winter.