for the leverage you do not pay an interest if you daytrade. If you close your position before end of the day then you have not borrowed any money. If you keep the money overnight you pay the interest as you correctly counted. you need to know your interest rate if it is per annum or per diem when you count it. Or you try it one day and see the rate on your report 🙂
you open an account with IB and only after that you buy DAS and request the account link through here to obtain the special offer for BBT members: https://bearbulltraders.com/request-special-das-package-for-ib-traders/
You can trade with TWS too if you like but it has its limints comparing to DAS. There are no risk controls in TWS and no hotkey scripts so you would need to do everything manually.