I'd say get a used setup locally in the 90-180 gallon range. Make sure it has an overflow. Then test your skills for at least 6 months to a year to see if you really want a 300g. Having a smaller cheaper tank will give you an idea about how you would like to have things setup and will prepare you to plan the setup of a large tank. Have you thought about water mixing stations, qt tanks, calcium/alk/mag dosing, auto top off? These are just a few things to consider. Testing your skills on a moderate sized cheap tank will help you identify design/setup flaws that you would prefer to do differently the second time around. No one does everything right the first time, your corals aren't going to outgrow a 100+ gallon tank in a year, and in 1-2 years you can set up the 300g and be great full that you are much more prepared to set it up the 'right' way.
Not to mention that this is your 8th post on here. You should read a lot of the build threads, check out the equipment forum, and look at the used equipment forum (once you get 50 posts) to see what kind of equipment you want to use. Over the next month or more you should try to learn as much as you can so you can make informed decisions to set up your tank. Read and ask questions because there are so many options and many different ways to keep a successful tank.