Level 40 comes when you hit 5,850,000 XP.
I found the best XP came from doing the challenge tasks. This helps as the reputation gain is the same as the XP you get.
The best reputation items for me came from:
- Mitz with the MiniCar, which gave between 32k and 36k
- Swekia with the Fancy Office chairs, which gave between 32k and 38k.
The XP you get might be different. You also might have to wait until you have levelled up all the clients.
Best tip that I can give would be to re-configure your factory to better suit which items give you the best XP.