The Raiden Alderaan release is a bug bounty release which puts a certain limit on the amount of tokens that can be deposited into a channel. This is to minimize potential loss of funds in case of bugs.
The global limit per token network is approximately $1000000.
The limit per channel side is approximately $1000.
W-ETH stands for Wrapped Ether and is essentially Ether that has been packaged to become ERC-20 compatible.
There are several ways in which you can create W-ETH from your Ether.
DAI is a popular stablecoin. Visit Maker DAO for further information on how to purchase it.