Should I be participating in these like-4-like systems or should I only be upvoting content I find valuable?

I find that I prefer communities that interact more actively than just vote systems. For instance, if you know discord there's lots of nice communities for that, especially: The Steem Engine, feel free to check it out.

Should I aim to gain more steem power or SBD? How does reputation play into this, and how do I grow my reputation?

If you're in it for the long haul, STEEM POWER is better, because that increases your influence as well as assists in the payouts of your own posts. And if you believe in the platform, that STEEM's price will grow, you want to accumulate more of that as well. You should work with the assumption that SBD will eventually be worth around 1 USD though if STEEM continues to grow. The higher the STEEM price is, the larger the payouts become / more SBD given out.

Reputation is all cosmetics, really. You gain reputation by getting upvotes from other people with high reputation. I hardly pay attention to this number, though it does correlate with influence. Each rep level takes much more effort to get than the last as well.