For regular office applications like "Microsoft Office - Excel, Word and Powerpoint... etc" most if not all i3 will be sufficient. However, if you prefer the system to run smoother and cut some waiting time on larger powerpoint files (>20MB in size), I would suggest getting i5.
Asus Zenbook UX31E-DH52B 13.3" Ultrabook ~Expensive, yet stylish
Samsung 300V3A-S02DE 13.3" ~Price in between and functions just as well
Fujitsu LifeBook SH531 ~Cheap alternative, that is bulkier