Before I ran into my casement window issue (other post) I was in the same boat, replacing 6 large steel sliders on a second house on my property. The siding was stucco, and like an idiot, I first called a friendly sounding window company from the radio. I suppose it was helpful, as like happened to you, they quoted me $15,500 to replace the 6 windows with fiberglass sliders. I told him that unless that quote included him giving me his F-250, he better get the F off my property. I realized that I would need to do this myself. I did a lot of learning talking to the guys at Home Depot and Lowes, but ultimately got my 6 windows for around $1050 - Milgard Style Line or something vinyl windows. The tricky thing was to make sure first the inside measurements fit, but what the Youtube doesn't tell you is to also consider the outside trim dimensions. In my case I wanted to have my replacement windows have a consistent reveal around the top, left and right sides, and weep down the oddly intact lip of the frame. Depending on your frame and exterior cladding, you should consider how the windows will fit on both the exterior and interior. In my case, sizing them down to fit inset meant I didn't have to add another 5 hours to my time grinding out the old window frames and could instead just throw away the old sashes and support bars. The proper caulking, low expanding foam, stainless steel screws, interior trim boards cost another $80, and I hired a helper for $20 an hour to make it go faster. All in all, the removal of old and install of new, came to a total of $1,300 and took a full day. This was BY FAR the most satisfying DIY project of my whole remodel, which probably wouldn't be the case if someone hadn't given me sticker shock.
I will say, I had it easy, single story, reasonable access, reusable frames, pretty level, stud construction to screw into, wide eaves so moisture barrier, while critical, was going to be tested less than in a house with shallow eaves. Still, amazingly satisfying and shockingly easy project to complete. Plus all the prospective tenants noted it and appreciated it.