Full time employment + Freelance on the side

Hello. First post on this site... and big grown up questions to ask myself!

I am in a full time employment, it's a secure pay, loving the job. However, more and more people have asked me to do design work. This turned into nearly regular jobs, in the sense that they keep coming back with more tasks. I have no intention of leaving my full time job, but do want to work extra in the evenings. I assume the right thing to do here is to set up a business as a sole trader.

A few questions:

1. When I register as a sole trader, do I have to pay NI twice (from my workplace + freelance)?
2. If I set up as a sole trader tomorrow, can I buy the softwares/computer I need for the job tax free?
3. What happen if I am not successful, in the sense that I get 2 jobs a year and make something ridiculously low (especially if it is less than the payable NI)

Thanks for any help or links to relevant posts, I am finding starting your own business not as straight forward as I hope it would be!

T. :bye