Should I buy Payday 2 now or wait for the Summer sales ?

Payday 2 is currently on sale for €4.99. The current sale will last until 23rd (check timer). There's a chance that it won't get renewed for Steam Summer Sale. This means that if you don't buy now, you're risking to lose €15. Sale bonuses are not worth this risk.

Also, the game has never been sold with a higher discount and that won't change during Summer Sale because of Steam refund policy. If the sale discount was lowered so soon, purchasers could refund and buy the game again (at a lower price).

Don't buy it. At all. Their DLC policy isn't worth supporting.

Doesn't really matter. The discount will last throughout the sales until the 4th of July. Besides, it's free to play until then so if you have your doubt, give it a spin for now.

Also, if you're planning on buying, just get a few of the DLCs, no reason to get all or most of them. I highly recommend the Gage Courier Mod Pack and Shotgun Pack. Easily the best and most essential DLCs for the game.