Blunt answer is you can't attract Lewandowski or Suarez at this time, so you are gonna take an expensive bet on some prospect, be it Lukaku, Belotti or similar level: players approaching their prime, physically strong enough to thrive in the PL, not the finished article yet and needing to develop together with the team. You can't replace Ibra with them, that's for sure. High risk, good but not exceptional reward then. If that's it, why don't use Martial and Rashford together with an experienced and proven 2nd tier striker like Vardy plus someone like James behind them?