The main issue isn't even your choice of what quality players you buy (though obviously you can get much better quality at the beginning of next season if you're promoted).

The main issue is that even the very *same* player that you're buying at the end of the season will be worth (and thus cost) ~15% less at the beginning of next season due to aging.
Even putting aside that you don't want to be buying players of that kind of quality for the next season, if you decide you want them anyway, it's almost always best to buy them at the start of the season provided that there's not much bidding competition for them.