A “manageable amount” is obviously subjective and will vary for each person based on things such as time available to dedicate to trading. Feel free to do your own research to find the right exchange for you. I tend to value user experience of an exchange over the amount of coins on it. Ultimately, what exchanges you use is going to depend on your own personal preferences. GDAX and Poloniex will provide sufficient resources needed to be a successful trader, so they are definitely a good place to start.
We will see Lightning Network dramatically increase adoption in 2018. That being said, it’s really hard to predict development cycles and we could see Lightning Network growing slowly this year and only hit its stride in 2019/2020. I predict currency coins that do not adopt Lightning Network will lose market share to Bitcoin. BCH, Dash, Pivx appear particularly vulnerable.

Only invest what you can lose. During the recent crash in January 2018, hobby-investors got burned. Reports of frustration and losses came at the cost of broken monitors, smashed laptops, and heavy monetary losses. While the rules are in more particular order of importance, it’s safe to assume that this is the most important rule, the rule to rule the rules. As soon as your money is converted into cryptocurrency, consider it lost forever. There is absolutely no guarantee you can get it back. Losses don’t simply come from dips in the market; extraordinary factors such as hacks, bugs, and government regulation can mean you’ll never see any of your money again. If you are investing money you can’t afford to lose, you need to take a step back and re-evaluate your current financial situation, because what you’re about to do is an act of desperation. This includes: using credit cards, taking out mortgages, applying for loans, or selling everything and traveling the world (as glamorous as that sounds).