Welcome to AA.
Canister filters are fine for use with fish only systems, but can become nitrate factories, aka nitrate timebombs when used in reef systems. LR
is the BEST choice and that seems to be what you are intending anyway. The LR
will take care of your biological filtration needs (1.5 lbs per gallon is a good place to start) but you'll want a good skimmer to supplement your LR
You mentioned that your cycle is almost done, which is both a good thing and bad. It's good because you are a step closer to getting living things in there, but also bad cause it may be a little more challenging now.
I say that because you are talking about incorporating a sump and that my be a challenge with a tank already full of water. It's always easier to do all your plumbing in a garage or outside and test it for leaks with freshwater and make adjustments from there without the spouse giving you grief for soaking the livingroom floor.
Also, you will also re-start your cycle when you introduce your LR
if it's not cured already. If you;re buying LR
online, it definetely won't be cured. If you are buying it from a LFS
, then it will have needed to be in their tank for at least 4 weeks to be considered cured.
If you place uncured rock into your pre-cycled tank, it needs to go through the whole process again to deal with all the die-off/waste the new LR
places into the system. Kind of a back to square one thing.
You could however cure the rock in sepererate containers, but you'll still be waiting on the rock before you can start stocking your main tank anyway.
Make sure that whatever you do, you do it slowly and don't rush things in the beginning... it's hard to wait for it all, I know from my own experience, but it is important that everything gets started right for the longterm success of your aquarium.