Easy answer for insurance is go online to a couple sites like Geico and Progressive and get a quote for the bike you are interested in. There are a ton of variables that go into insurance so it hard to get an idea what it will cost without doing the quote yourself.
Ninja 250 or 500 are great starter bikes and are good bikes for general use also. Speed triple is a completely differ league. You may have the skills to step up to one but it will be a huge jump from your previous experience. Ninja 500 or GS500 may be a good bike for you to start with then step up after some experience with a bigger motorcycle.
MC licenses vary by state, but in general you can get a license in several steps.
With a valid drivers license you take a written test and get a MC learners permit. You can then legally drive motorcycles with some limitations (usually no highway or after dark).
Once you have the temp you can take the skills test and then get your permanent endorsement.
An excellent suggestion is to take the MSF basic riders course even if you are an experienced rider. You really can learn alot in the course and in many states you can take the skills test at the end of the course.
Also look for used bikes to start with especially if you intend to upgrade soon. Lightly used Ninja beginner bikes like the Ninja 250r are easy to find on the used market and you can usually recover much of your cost when you resell. If you buy new you are usually luckly to get half that cost back when you change bikes.
Craig