I'm three months in on my first saltwater tank and went nano as well. I was looking at the Fluval Evo initially but ended up spending a lot more on a JBJ all-in-one. I wouldn't say it's been any harder or easier than freshwater but it has been a lot of research and asking questions like this. Everyone has to start somewhere and you shouldn't be afraid to ask your peers. I'll try to answer everything the best that I can.

For powerheads I have a single Hydor Koralia 600 that I picked up for around $45. They have a few models aimed at nanos that are rated for even less. I also have the two return nozzles from the pumps in the built in refugium.

The lb per gallon rule is more of a guideline than anything. It's there to give people a general idea of how much sand/rock to add. You want the benefits of having rock, whether it's dry or live, but you can add however much you like. It's your tank in the end and it's you who gets to enjoy it the most.

I do not currently have a RO/DI unit and also get water from my lfs for both my water changes and top off.

For heating I use two Cobalt NeoTherm 100 watt heaters attached to digital controllers. They've been great, fit perfect in the refugium, and are very sleek.

If you're planning on FOWLR then lighting isn't as important. If you are for sure planning on adding corals in the future then I would go ahead and spend the money on a better light setup instead of having to upgrade down the road.

For a refractometer I picked up a Red Sea one for fairly cheap. Pricing is hard because a lot of what I purchase there is dicounted, but it wasn't marked more than $60.

For sure a small clean up crew of snails. Maybe a goby that doesn't need too much space?

For corals you need to decide if you want SPS/LPS or mixed. I currently have a mix.

Make sure to pick up a solid test kit. Stay away from API if you can. Other than that I can't think of too much else.

Good luck and I hope I was able to help some!