So let's talk about the Fleet Arena Store.
Your mission here is to get all of the Fleet Store-exclusive characters & ships & then move on to collecting zetas, nothing but zetas, forever after.
That said, some people prefer to avoid certain characters and others (like me) are willing to use this store for certain things that aren't quite exclusive to the Fleet Store but are hard to get elsewhere. I'll create the list that I think is best, but I'll also put an asterisk next to toons & ships that are technically possible to get elsewhere...except Grievous and things that are available elsewhere only in the Guild Events Store, where the currency is just way too valuable to use for anything that can be purchased in this store. Even sacrificing some zeta mats is worth it to get the exclusive characters & gear from that store.
Fleet Arena Store: 1. General Grievous
2. * TIE Adv x1 (Vader's ship)
3. * Slave 1
4. Rex's ARC-170
5. * Phantom II
6. * Ghost
7. * Bistan's U-Wing
8. Chirrut Imwe
9. * Scimitar
10. * Gauntlet Starfighter
11. Fulcrum
And then, of course,
12. Zeta mats forever.
possible side quest?
13. Take a break from zeta mats for toons you haven't finished that you would otherwise have to buy in the Guild Store
14. Back to zeta mats forever.
The most common place something can be found other than the Fleet Arena Store (And Guild Events Store which you should NEVER use for any of these things) is Fleet Battles. But there are a large number of things you want there, and the hard-node limit of 5 attempts per day makes this a slow source for things as far down the priority list as, say, Bistan's U-Wing.
And yet, BU-W is actually a great ship, it's just that to make full use of it you need a number of other things to come together, and so it's not a huge priority early in the game. Phantom & Ghost are available in Regular Energy nodes, but, again, to get the most out of them, you want the Han's Falcon, which you can't get without all 4 Bounty Hunter ships... which is going to take a while. In the meantime, the Geo ships, which you can get easily in the GW Store, are going to tide you over as your arena squad while Slave 1 helps you get the Falcon and Slave 1 + TIE Adv x1 are both useful for some dark side events like boosting Tarkin's Executrix so you can get the zetas flowing in the Ability Mats fleet challenge.
So I fully recommend using the Fleet Store for everything listed here, but if you have the chance (and the energy) to fit in some farming of some of those other ships or toons that have an asterisk, go ahead and work on them some so you can reduce the amount of precious Fleet Tokens you spend on them.
Much more arguable is Plo Koon. I used fleet tokens for PK and I don't regret it at all - his ship is absolutely fantastic for starting players and Plo himself is super-easy to gear compared to other toons (in early game everyone seems hard, but trust me: the gear you need for Plo isn't stuff you'll regret spending on him). That said, he is available elsewhere fairly easily. It's just that when I needed him I was really busy with what were time-crunched farms for an event (I don't even remember which event). Could you spend Fleet Tokens on toons like PK or a couple others? Sure, maybe. There's even an argument to be made that the gear you get with Guild Tokens is more valuable than the zeta mats you get with Fleet Tokens, so buying your Clones here is better than buying them in the guild store.
I get that. And truthfully in the long run you'll catch up on zeta mats & begin to have a bunch of mats stored up in advance of when they're needed. When I unlocked SLKR he required 6 zetas, which is 120 zeta mats. I spent them all and had 250 left in the bank.
However, where you are in the game, you need all the zeta mats you can get. So I would recommend just doing what we've outlined above, and then if you catch up to your zeta needs and still haven't finished some farms from the Guild Store that are available here (like Clones, but not only Clones), then take a break from zeta mats for a little while to get specific characters, then back to zeta mats.
FINAL NOTE: Some of those things are rare. Grievous, the TIE Adv, Bistan's U-Wing, and Rex's ARC-170 all show up randomly and not that often. If you're farming one of these and you have Fleet Tokens start to pile up, keep 2,000 to 3,000 in the bank to make sure you have plenty in case you go on a lucky streak with appearances of your top priority and then spend anything more than that on the next item down your list. There's no sense saving up 15k tokens while you're working on Grievous when you could have used it to finish the TIE Adv already.