1st saltwater tank.

[searay]

Hi everyone.
I'm new to this forum and to salt.I have had freshwater tanks for a long time and finally decided to do a FOWLR aquarium.
I have done a bit of research and have started to aquire equipment.
Some opinions would be great.
This is what I got so far.
-I bought a used 120 gal 48x24x24 starphire glass tank with corner overflow,stand and 70 gal sump and mag 18 return pump.
-Ordered a SWC extreme 200 skimmer
-Ordered 100 lbs eco rock
-have two 24" glo t5 h0 light fixtures,heater and tunze auto topoff for sump.
Questions.
I removed the baffles in the sump so I can custom it for the equipment I have and a refuge.Sump is 40" x 20"w x 18"t Ideas?
Can I downgrade from the mag 18 to a eheim 1262 ?
The drilled return and overflow holes are drilled pretty big.Will this be an issue.?
Tank will be in the living room and not sure how loud the mag 18 will be or if I need that much flow for a fowlr.
Want to do a sandbed and a glass top as a wrasse might be on my list.
Stumped on the sand.How deep and what kind?
Was thinking some type of caribsea brand.
Hints on cycling and adding live rock to seed would be great.
Still going over the fish list so will have more Q's if you folks don't mind.
Great site.
Thanks
Rob