You have good questions, but you need to ask them from a local contact who has more facts than you should post on a public site.
Last I knew, banks would not consider rental income until you had a year or more of it collected, so your rental income will not be useful in qualifying. When it does count, only 75% will be added to your income, to offset vacancies & maintenance. Now the rules change, so double check this with a local lender. They don't charge by the question, and they can examine your income, assets and employment to see if you will qualify.
Don't commit to the rental until you are able to verify you can make your purchase.
An ARM should only be considered if you plan to sell the home around the time of an adjustment. ARM's now qualify you at the fully indexed rate (the highest rate it may adjust too) so it may not be helpful anyway.