u can try upwards solutions. they are pretty good. wether or not its easy to replace depends a lot. if you take a good frame you will need to integrate an anchor system in the concrete. then the windows can be made precisely to order. installation is quick one, two day tops. problem might be if additonal work is needed to the fit the frame then you need to take out the old windows, get the work done, get the openings measured and then custom make the windows for it. so you have a period of no windows (or maybe they can put the old ones back for the time beeing - not sure). best to be done with a major reno job.

double glazing is efficient but if you really want to soundproof to need to "up the ante" - normal noise shut off, double glaze is enough ( i am happy with it).

depends on size, quantity, quality and fittings choosen. 400 sqft of windows can range from upwards of 40k to 100k easily. it will very much depend on the window frames and the level of glass you choose.