Players I feel would take us to the next level are:
Defender: Lescott (Free) or Richards (£6m) or Martins Indi (£6m)
Midfield: Mo Diame (£3.5m release clause) & either Alex Song (£12m) or Yann M'Vila (£9m)
Stikers: Both Michy Batshuayi (£6.5m Release Clause) & Alexandre Lacazette (£12-£15m)
The 2 strikers are the most important and I feel these are much better options for the money than a lot of the suggestion made on other comments.