I disagree that FOWLR is boring colour-wise. My tank started off fully intended to be be just a FOWLR and I have some lovely purple coralline algae on my rocks, plus you can add things like barnacle shells etc. There are also inverts like feather dusters that come in a few colours which can add a similar feel to a coral tank without actually needing light as they're filter feeders. The only bad thing is that coralline takes forever to grow, and it's rare that you see any live rock at LFS with much on, but as in all things marine, patience is key.
You'll notice I said my tank was originally intended as a FOWLR, as I've just bitten the bullet and started to buy some soft corals. The good news with soft corals is that they're generally very easy to look after, requiring nothing more than some reasonably decent lighting and preferably a timer to run it. Your regular water changes will keep them topped up with everything else they need, as it's in the salt mix. You mentioned you've had a planted tank with CO2 before, so I wouldn't have thought soft corals or even SPS would be a problem for you at all. Soft corals are a great starting point. Have a look at some zoanthids, as they're very easy to keep but come in some wonderful colours.
Marine tanks seem scary with all the extra stuff you have to consider with fish, corals, plus additional equipment, but if you take it easy and slowly it's not that bad. I've had my marine tank running since May, and I'm still not up to my full stocking with fish, and I've only just this month added my first coral.
I've seen plenty of plug and play tanks with weirs, but I expect you'll struggle to find one with a sump. Plug and play tanks are more targeted towards folk who want a pretty tank in their living rooms, so the benefits (and expense) of sumps such as bigger better protein skimmers, refuges, the ability to grow chaeto etc just don't really appeal to them, whereas for us aquarists,it's almost a necessity. If you're looking for an all in one kit, I'd keep looking at Gumtree and eBay (there is an option to sort search results by distance to you) for a second hand setup.
My tank is 155L plus the sump, which seemed massive when it was a freshwater setup, but pretty small for a marine tank. 90L, even more so. Filtration isn't really an issue, as your live rock will provide ample biological filtration, your protein skimmer will do a good job of removing waste. Unlike in freshwater tanks, you also get the anoxic bacteria that convert nitrate to nitrogen gas, which can be encouraged by carbon dosing (which isn't something you should be looking to do any time soon, but know that it exists for future reference). In a freshwater tank, your stocking is mostly limited by filtration and water quality. In the marine world it's different, as while they are more sensitive to water quality, there are many more methods available to you to maintain it. The main issue with stocking a marine tank is that there are very few marine fish which aren't at least a tiny bit aggressive or territorial, and adding too many doesn't really end well. The good news, however, is that there are a lot of fish which do best in singles or pairs, so while you won't be able to have a lot of fish, you don't have to waste stocking space with several of one type like you would in a tropical tank.
