How much topsoil? Boxblading to level will certainly rip out and kill many, if you do that, and adding a lot of top soil would certainly bury others. The process of smoothing it by machine will also do damage, so that's good.

If we're talking six inches, then I wouldn't expect to see too many coming back. They're entombed and I buried grass and weeds like that when I added raised gardens without any issues (even four inches went fine with only a few sprigs making it through in known thin areas).

If an inch, expect to see plenty of weeds popping right through as if nothing ever happened. Some will certainly die as their crowns are buried, while others adjust.

If a quarter inch, survival rates will approach a hundred percent. One can always add a quarter inch of soil without too many issues, including on established lawn and most gardens.

The short answer is no. There are too many variables involved to rely upon final grade and top soil as an effective weed treatment including any potential weed seed cache that comes in the top soil.

You might want or need to regrow your weed lawn over the summer after final grade is done and kill that before hydroseeding in the fall when it is a better time to grow lawn grass from seed.

When I was in college I worked for my brother in law doing final grading for apartments and houses. I strongly suggest you do not presume to know how much soil you need. Before bringing in topsoil, you should consult with a (three) professional landscaper(s). They might recommend removing topsoil to get your drainage right.

Whatever you do, do not let anyone rototill the soil. If you have high spots that need to come down and low spots to fill, a professional would use this tool...

It's a box blade on the back of a tractor. This job cannot be done efficiently using a Skid Steer or a Bob Cat. The guy on the tractor can do an acre in a morning. That would take a Bob Cat a week to do.

Where do you live and what do you plan to plant/hydroseed? And why hydroseed? Are you on a slope that cannot be walked? Hydroseeding is kind of expensive.