In terms of reusing water, I normally say it is a waste. Why carry over nutrients that you will want to export later on? There isn't any beneficial bacteria in the water column, it is in the sand and rock in the established tanks.
In terms of the sand and rock, I would reuse it. I would rinse the sand in the old tank water to ensure there isn't any excess food/poop in it before the transfer, back to not wanting to export nutrients later on.
In terms of your canister filter, it is doable. Normally these are frowned upon in the SW
world for the exact reason you listed. But, if they are broken down and maintained well there won't be an issue with them. I have never attempted, but I would suggest breaking down and cleaning weekly, just as with partial water changes.
If you choose to go the route of a sump, you can get HOB
overflows for your system...but drilling is better as it is gravity driven. The HOB
overflows can loose siphon...though there are units out there like the CPR overflow that works to prevent that with the assistance of an aqualifter.
In terms of stocking, you are correct with being limited by the size of the tank...but there are several "nano" fish out there that can suit your needs. My first tank was a similar sized bowfront I bought from Petco. I had some clowns, a royal gramma, a chalk bass...just to name a few. I suggest checking out liveaquaria.com and doing some research as to what you like and if it is all compatible together.
Good luck, ask whatever you need and we'll try to help.