Considering how easy it is for non-EU players to currently get work permits I'd guess that it wouldn't affect many signings, especially not international players, who the PL clubs can obviously afford.
It might mean a reduction in the number of foreign players, so perhaps less stacking of players in certain squads. As for investment I really don't know, politicians don't really know, they each spin it whatever way they desire. Potentially it could put off overseas investors in the short term.
It might be interesting to see how well the pound does against the euro in the next few years, were it to happen. As it is it's doing poorly, and there is no let up in Premier League buying of players from Eurozone countries.