There is no doubt that the current owner of the club can change the team quickly but return to the problem of financial fair play but even so the first step that must be done at least is to recruit a great coach because according to rumors, Newcastle are targeting to get Antonio Conte as their new coach. I think it's easy to achieve just to stay in the EPL and get out of the relegation zone with the players that are there, because the most important thing for Newcastle is to have a great coach. We can see how Thomas Tuchel made big changes for Chelsea upon his arrival well with that case, at least having a great coach was enough for Newcastle out of the relegation zone and stay in the EPL.
Staying up in EPL is never as easy as it seems. If you spend long enough without European action,,, then you attract less and less players and then you become a mid table club that only attracts either young players on loan looking for playing time or older veterans who only stay a few seasons.
Money is a dangerous thing and Leeds found that out the hard way. If FFP works for Newcastle then they will challenge for Europe for sure but very few clubs can do it without breaking the bank long term. This is Spurs, Liverpool and Arsenal at the moment and see how much they struggle to always remain top.