Simple advice: 40b fowlr - What equipment to buy?

I just had to leave town on 12 hours' notice, and came back after 3 weeks. My two clownfish were taken to my LFS, but the tank sat in the dark since the middle of January. I am using this moment to do two things: Make sure I have appropriate equipment and a plan for purchasing more, and clean everything as well as I can.

I'm quite new to the hobby, so this is a full list of my setup:

1. 40B tank
2. HOB filter
3. One powerhead (can't recall the GFM offhand)
4. RO/DI water filter
5. non-LED lights with a simple on/off switch, no automation
6. About 2" of sand at the bottom, under 40 pounds of rock

I currently have two ocellaris after the aquarium ate my other two fishies. My current plans are to have more simple fish - e.g. a mouthful of chromis - but coral is out of scope for this tank.

I have been given a Rena Filstar XP2 that is missing some parts, and while looking for replacement parts I found that I could just buy a brand-new canister for a similar price. Then I found out that a canister wasn't entirely recommended. So this leaves me with some specific questions under the "What should I buy" heading:

• Should I buy a canister filter? If so, what is a reasonable size?
• Should I buy a skimmer? If so, what advice can you give me?
• What about buying both?
• Can I make do without a sump? I don't think I have a good place for one.
• I think I'll look at lights later in the year. What recommendations do you have for a tank like this?
• Are there any other recommendations?

I'd like to have a solid idea of what I need to buy through the rest of the year so I can plan accordingly. Thank you for your help!!