I have not found the limit of trading for Bittrex or maybe I missed something.
But for a minimum purchase in one trade is worth 50k sat.
Yes, you can directly use Bitcoin in your account wallet. But make sure that your deposit is confirmed Bitcoin network.
For this I do not understand what you want, why do you want to withdraw or transfer directly after making a deposit to an exchange account. but it's up to you and surely you can do that, But in bittrex they have daily withdrawal limits for unverified accounts