Is $3,500 to make an inclined structure on the sunroom roof a fair price?

I am a first-time home buyer and my sunroom roof is leaking, affecting the lower point of the ceiling seams, though there's no active dripping. I've seen old photos showing similar damage that was previously painted over. My contractor friend believes water is pooling on the roof due to a lack of incline. I'm planning to make some improvements myself, including installing a bigger 5-inch gutter and sealing the top seams on the roof. I'm hoping to seal it and have it last a long time, and since my house is older, I want to avoid spending a lot if possible.