I don't advise you to invest only in bitcoin. There is too much risk if something suddenly happens with bitcoin. Break your money into several parts and invest in different cryptocurrencies. It also matters for how long you intend to invest your money and what you plan to get from this.
Don't be silly. There's a fuckton more risk in most other coins. Anyone putting their life savings in some shitcoin in case Bitcoin dies is a moron. Have a play with a small proportion by all means but Bitcoin is the only one I'd be certain was still here in 5-10 years.
As for the original question it depends on how big a proportion of your savings $15,000 is. If it's all of it then that's a really bad idea. If it's 1-20% that's more palatable. But no one can tell you whether it'll be the best move you'll ever make or the worst or the most boring.