Minimum staking value is 0.1 Strat, as defined in here: https://github.com/stratisproject/StratisBitcoinFullNode/blob/master/src/Stratis.Bitcoin.Features.Miner/Staking/PosMinting.cs:

"
        ///

        /// We don't stake coins that are smaller than 0.1 in order to save on CPU as these have a very small chance to be used
        /// to generate a block anyway.
        /// https://github.com/stratisproject/StratisBitcoinFullNode/issues/1180"/>
        ///

"

Rewards depend on the total amount of strat being staked on the network. 1 new Strat is minted with every block (each 64 seconds). So if EVERY Strat was staking at once, then the rewards would be ~0.5% per year. As things stand, about 1/4 of the Strat supply is staking, so to work out the staking rewards you just have to divide 0.5% by the share of the network staking, i.e. 0.5% / (1/4) = 2%. So at the moment, if you're staking 25,000 Strat 24/7, you will be receiving roughly 500 Strat in rewards per year.

Staking does also receive fees associated with transactions contained in each block. So rewards will go up a little with overall network volume. As things stand, these are very small compared with the 1 Strat minted each block.

Staking is pseudorandom, so all of the above ROIs are averages. If you want to dig deeper into how the staking works, that above GitHub page tells you everything you need to know.

You can use ARK Desktop or ARK Mobile Wallet to access your wallet, just use same passphrase you did with Web Wallet.