In long:
Live in northeast. I am going to be replacing a picture window that has 2 storm double-hungs on its sides with a bow window. Also replacing same window config downstairs with new picture/double-hungs. I've been getting various recommended contractors come in to give estimates.

Besides window choice which is a separate decision for me, what technical questions should I be asking the installer?

I am a DIY, but never have done windows. Will do individual double-hungs around the house by myself eventually, but for these big ones, I want an installer but don't know what to ask besides:
-Warranty info from him? Examples of previous jobs? Is he insured? Removal/disposal of my old windows included?

For example, one guy came in and said he'd prefer to do a replacement because it'd be cheaper to install because of my vinyl siding and having to do more trim work, etc.. But another guy said he'd do new construction and used technical terms I wasn't really familiar with at the time.

You can use whatever terms and I'll research what they mean. Tnx!!!!

Other than the support for the bow being structurally strong enough, the simgle most important factor is the use of window tape, (Vicor or some other brand). This must be installed per instructions, overlapping like roof shingles. In addition, the contractor may use silicone to make sure there are no gaps for water to seep in. A great percentage of window problems that leak water is not the window but how it was installed.

If it is new construction or you have all the siding off, absolutely use a house wrap like Tyvek, again following instructions.

Lastly, the window must be installed level and plumb then shimmed (top at both sides and at the check rail for DH. The shim should be snug but not forced in. This is critical on DH windows. You want the sashes to slide up and down without being too loose or too tight. Shimming is critidal here.

If you see the contractor putting the windows in and just nailing the nailing fins without first checking level and shimming, you will have trouble. This could be especially troublesom if he takes the sashes out to reduct weight and installs the window frames in the same manner by just nailing the fins without leveling, shimming.