Suppose someone is interested in contributing more than 50 ETH? How do you enforce that since people are just sending funds from their wallet to yours? They could indicate that they want to send 50 ETH, but in reality only send 5 ETH. You would still have to honor sending them tokens as well. Isn't this just a "pre-sale"?
Whitelisted participants' addresses will be added to contract and they will be able to send ETH before the public sale will start.
And I think 50 ETH limit is the right restriction!