Hiya ...have you been doing any nails since 2005...?
I think you can make a living doing anything if its sells....and to make it sell you have to be 1) good 2) offer more than the others in terms of proffesionalism..products..etc and 3) be prepared to work hard.
you may benefit from doing a refresher course....as to costs, its very hard to say. You'd need to draw up a list of everything you need and go from there.
Tax is tricky too as it depends on how much you are earning...(and if you have another job too) not 100% sure but if your profits are under 6k (ish) you don't get taxed...so for the first 1-2 years you should be ok..but then after that and if your earnings increase that amount, you will have to pay some tax.
Also insurance...its not much .. think mine is about £35 for the year