Hiya,
That's a tough one. In reality it is hard to just 'go for it' if you have to support yourself entirely, but on the other hand, if you take a job as well, you never really get the chance to go for the freelancing 100% as you can get distracted very easily. Plus, clients like you to be businesslike and do expect you to be able to answer calls and emails during the business day. The first thing is to assess your finances and see how long (or if at all) you could survive on less, or no, income. That will make the decision for you. Ideally you need a three month stop-gap at least.
I never really did a business plan, I just worked out how much money I needed a month, how much I could reasonably charge for what I did, and then how many hours I needed to work to support myself. Once I realised that was viable (as in, there were actually enough hours in the week!) I knew it was worth a shot
From there, it's about getting as many people as possible to know you are out there. Business contacts from your past life and friends/relatives/colleagues are a great palce to start. Have a look at
http://www.freelancecopywriting.biz/...eMarketing.pdf which is an article I wrote for this site about how to get started, and some things to think about. There is also a version of the article on here under the 'freelance marketing' section.
Feel free to ask some more questions after you've had a look!
Cheers,
Gill