Is it good to just buy BTC & ETH @ Coinbase and leave it there for investment?
I think the fees might be pretty high or is it not?
And after I buy it, should I leave it there or send to some of my created wallets in Electrum for example?
I'd just leave it there for the investment. Cryptocurrencies are blowing up all around the world. Having investments in them is a good idea for sure. hold onto them for a couple years and you'll be able to retire.