If the person is in your area - meaning you know where they live, you can sit and have a coffee together, then I might consider this solution (after checking/confirming previous work they've done).
If the person you have found is based on a skype and email chat, then I would avoid. Especially if they want any payment up front. Bitcoin is like Western Union - once you put your money in the pot, there is no getting it back. You have no idea this person is who they say they are and whatever proof they share has question marks on it. You're unlikely to get an invoice that your tax man will accept too.
I don't use the freelancer website - but I do question someone who uses the website and does not want to use their payment methods.
I would also think twice about sharing credentials with your webserver, check any other work they have done (and confirm they've done the work and not just take their word for it).
Best of luck