I don't remember where Rare Earth Elements come from precisely.
I think there's a trade deal relatively early on that lets you buy them (Edit: I think pledging to the Mutant Alliance, which is arguably early but not very early. Technically optional). And I think you get them from Church equipment, possibly by disassembling their stuff.
Plantation simply has negative upkeep - it makes you money every month.
It's also a good placeholder for two other types of farms you can make soon, which produce somewhat specialist resources.
Ehhh... gosh, I'm uncertain about all of this, but my recollection is that base attacks generally choose a random base regardless of how many you have, so it can actually help distract from the main base with the priceless facilities. But being spotted by certain factions, particularly those with "ground" patrols (you'll see them), can lead to attacks independent from that general retaliation roll.
tldr; Yes, you very much need to worry about attacks. Be aware that there's an open "sewer" underneath even unused tiles, only the security-corridor facilities fence it off at all.
WORKERs draw a salary but increase your storage capacity very significantly, they're a valid route particularly if you're avoiding slavery (which also increases storage). LIABILITIES is relatively new but I think represents things which have upkeep, like workers and advanced medkits.
It is bogglingly deep.
Is interrogating civvies a good idea if you're hurting for infamy?
Personally I can't tell if any of them make a big enough difference...
You get the small amount of points for interrogating them, as well as from anything you learned (I think you get both). Also, when you capture that type of civilian in the future, you'll gain 10 points instead of losing however many points. It's always a good idea to interrogate one of each type that gives you negative points (you may want to wait a bit on some that give you positive points for a capture). There are also some topics you can learn quickly from a civilian, or somewhat more slowly from a higher-value capture. Researching lower-value targets first means you'll get better tech sooner when you research everyone else.
I *think* you only get infamy credit for new research items, such you don't get the credit for researching the civ (or general or whatnot) each time. It has been a while since I did that test though.
It's almost always a good idea to research/interrogate them anyway, unless they have some special materials worth robbing.
Final edit: I feel bad for calling the Hellerium pool a sewer. It is literally the source of our power. It's also a frustrating slog that invaders love to wander into and waste time in.