Hi Sam,
Welcome to Bigger Pockets! It is great you have taken the decision to start out in Real Estate and considering ways of building your wealth one step at a time. Getting a license will definitely open a lot of doors for you, so don't hesitate about it. The great thing about Real Estate is that there are so many options and opportunities no matter if you do it full time or part time.
Regarding your questions:
1. You can't jump straight to a broker license. A broker candidate must complete 8 approved real estate courses. In addition, you must also have 2 years of full time licensed real estate experience within the past 5 years or the equivalent. Equivalents include a 4 year degree with a major or minor in real estate from an accredited college.
2. As far as I know, depending on the states involved, in most cases you can do that but have to still sit for the exam in the state you want to get a license from. For example, I have a CA license, If I want to get a Nevada license, I do not need to take additional classes, but just just sit for the Nevada's exam.
3. I have completed the Allied School CA RE course and I am very happy with them and how their program prepared me for the license. They give you all the books and online access to the student materials and tests, DVD, Cram Exam student book. Once you have gone through the books you can access online their final preparation test. They suggest that you access that test right before you sit for the exam because it is available for a limited time. Study well all questions from that test and you will successfully pass the exam.
When enrolling in their course, ask for their add-on certificates such as A Green Specialist, Short Sale Specialist, etc. for no extra charge.
Let me know if you decide to go with Allied, and I can refer you so you do not have to pay full price.
A real estate agent I have known for years has gotten his license with the help of Premier Schools - another accredited online school. He also renews his license with them every time.
Good luck with everything !
Evelyn