At what stage would you level up?

If you go to level 15, all you see is beach parts (8 of 10 slots full of them). Thus it becomes harder to fend for expansion items.

Land expansion still comes, but it gets crowded out a lot more by the larger variety of items available in the store as you go up. I would not go past lvl 18 if you want to fully expand the map in less than 6 months (took me slightly less than 2 months, I camped at level 12/13/14 for a long time).

You can open the Airport and VU without going to level 15. Just use population boosting buildings to get the required threshold (education - university, entertainment - ferris wheel, transpo - helipad)