Sorry to highjack, but im in a similar situation, although with a rio 240 so a smidge bigger.
My main question is about RO. Is this essential?
Depends on the quality of your tap. I did it on tap for several years, but then the tap quality degraded rather quickly and gave me a ton of pH/KH problems in my tanks. If you have a >2dKH, which is really quite common, you can pretty much write off using tap in anything but an emergency situation and will need RO or RODI as the staple water supply. While you can chemically treat for most things in tap, carbonates are a bear and salt mixes are not designed to go into water with substantial carbonate content, and you will actually see CaCO3 that looks like snow precipitate out and then stubbornly refuse to dissolve. Once that happens, your params are not what the salt manufacturer intended and the effort to tweak things back to being ok is substantial.
The other big tap caveat is metal content. If it tastes like sticking an old metal spoon or a coin in your mouth, then you can also write it off, but most people don't have tap that bad. I did live in a house with tap that bad once but it was questionable for drinking quality as well because of that.
i have got friends who assure me marine is just as easy as tropical - just add salt as well... but im not really that sure??? same about skimmers - my friends dont use them, but i suspect they might be neglectful.
They may have super-duper quality tap or, as you said, the conditions may be sub-par. If they're keeping a fish-only with just a few hardy fish they might not see issues from using bad tap (assuming it's not REALLY bad tap). I would be rather surprised though if they were successfully keeping a healthy reef with anything other than really good-quality tap or RO/RODI.
Also, i dont really have space for a sump and also dont really want to drill my tank, so i was hoping an external would be OK, but i may do a bit more research on that one.
An external does not substitute for a sump. All it does is give you an out-of-tank place to put things, which a sump also does, but the water volume and filtration capacity difference between the two is pretty big. That said, there is nothing wrong with using externals without a sump as long as you maintain them properly and are very careful about your stocking.
If i go ahead, i suspect i would do a FOWLR because i am a bit scared of keeping corals - i dont think it would end well.
My initial setup would be lots of rock and a few fish, and take it from there.
Sounds good. You also don't have to commit to a reef right off the bat. Many people start FOWLR and then add a softie or two down the line and may go full reef eventually. The main thing is that if you think you might want to go for corals at some point and don't have a light fixture, you might want to just go for a coral-ready one so that you don't have to buy another one later.
