Yes, why not invest?
The highest price of ETH in history is close to 1500 dollars, so it is 500 now. If it were me, I would invest all the money.
Yes, of course. Now is the best time to invest ethereum while at it's unstable price because I'm probably sure that it's value or price will get higher than before. Hold it in long term. Ethereum will recover, just wait and be patience enough to handle this kind of changes. You can invest ethereum now.