Welcome, I'm not that far ahead of you in the journey to the Salty side but can answer some of those questions.
As I understand Marines are run lidless by most people to improve the gas exchange at the surface of the water, although my FOWLR that is just being set up will have a hood as there is a blinking huge gap at the back.
If you're setting up a sump you can bin the 405 as the sump will have the skimmer and whatever else you want to put in there (read journals and make your choices they range from very simple to ridiculously involved) and the LR will be your filter, so you just have to make sure your blasting enough water at the rock with your powerhead so you are spot on with regard to getting more of those
With LR patience is the key, I spent 4 weeks just going through aquarist classified, fleabay and the like every night and eventually hit gold on another site by someone breaking down a mature tank in a hurry and ended up with 75kg of rock with loads of freebies for £180 once I had sold the surplus on. Be patient and keep looking.
Don't use ocean rock it is too dense and won't act as a filter even when its been in the tank a while, if you use dead live rock it will take a while to get sorted, it will have a cycle of indeterminable length and will probably whiff a bit, IMO you would be wise to wait for a good tank breakdown and pounce.
I have a bit on RO units on my Godiva journal shows it plumbed in and I think there is a thread on there about where I got mine, don't worry too much about the Gallons per day rating mine is 100GPD and does 25l in a couple of hours, but even a 50GPD will give you enough with a bit of planning. It's just the initial fill that takes time.
Don't use the play sand it will look horrid IMO and with powerheads it will be stirred up something cronic, just get some argonite sand/coral sand. It will soon be crawling with life from your LR. Should be able to get it for about £25 for 20kg and in that size tank you will only need about 10kg if that.
Powerheads are basically an electrically powered fan that blows the water about there are many options from the lovely, slimline, unobtrusive (but expensive) Vortech MP40 to the cheap but very capable Newaves, they are directly powered to the mains all plug and go, except a few that work off of 12v wavemakers, the options are endless so best get back on the net.
Taken me so long to answer this I'm probably behind Woody with an answer now and this is all redundant.