Adding to what Jerry's just said I had a 150W flash head mid 1970's it was just feasible to shoot full length model shots at about f4 through a diffuser. A couple of years later I had constant access to a few larger 1.5k & 3k flash packs but bought a pair of 500w heads for portability.

Now I still use those 500W heads alongside my main 800w head, but the 800w head gives me far greater flexibility, the old 150W head just gets a quick test every few years now. If I need more power I borrow or hire as needed.

Flash heads usually have a good lifespan, I'd guess my Elinchroms are 30+ years old now and still perform well. So get the best in terms of power you can afford.

Ian