It's a guess but ALL name brand vinyls should be about the same thickness and quality. The difference is in the design, look. There are more expensive vinyls that resemble cedar shake and other materials that look more expensive. Homedepot vinyl, I think, is thinner for lower cost and such but as far as longevity and impact resistance, probably are same. Depends on the kinds of impact but nobody assumes huge hail storm and such. Then all bets are off. Cheaper, thinner siding could make the difference in how it looks later. How the joints lay flat on eachother, eg. Ask your installer for the brand and ask again here or search yourself. "(name) review" and such search should bring up pages of opinions.
You can keep the old siding and it is another barrier against the weather, but it is another material that will hold temperature. Ideal would be to remove it and add insulation. But with inside renovation like yours, no return on your effort and money. All flashing must be done as if the old siding was your sheathing. Also who knows if old siding was properly flashed? Now is the time to do the work properly instead of finding out later.
