I am at work and don't actually want to do work so here is a lengthy response lol
Your lighting entirely depends on your fert schedule, CO2 distribution, plant mass, etc... Contrary to what all beginners believe there is not a 'magic number' for any of this crap. Adjustable lighting is key here. Make it so you can raise or lower your T5 ballast and tweak lighting as you go.
START SLOW. Too much light while your CO2 /ferts/plant mass are not where they should be will cause a lot of algae and a LOT of headaches. It is worth it to be patient.
You 100% need a filter. I would just use the canister you reference as long as the flow is not too weak or strong.
Do not use activated carbon. IMO all you need are some layers of foam (coarse – fine) then put your bio media on top of that. It is by far the most efficient way to pack a canister. This video explains how to properly set up a canister (FYI most manufacturers recommend to set it up THE WRONG WAY): https://www.youtube.com/watch?v=szrUlHTW5Zo
Substrate / Fertilizers:
For substrate I recommend a dirt tank capped with sand or something cheap like Black Diamond blasting grit. Either that or splurge on the ADA AS. Never used AS but people say it has a steep learning curve.
Fertilizers get dry ferts. Do not bother with liquid crap. Get a few pounds of KNO3, KH2PO4, K2SO4, CSM+ B, and maybe some Iron Chelate. They will last your little 12g forever and will cost you like $20 - $25.
I did a dry start with HC and it was… weird…
I did the same thing as you. Jumped into high tech planted tanks headfirst and all my HC browned and died off within a few months. My tank was a 75 gallon so it was more difficult than your endeavor but I will warn you that HC is VERY DIFFICULT. It is called a DRY start for a reason. Standing water will grow mold and your dry start will fail. Puddles are NOT GOOD during dry start. You want the tank to be humid and moist with absolutely no water above the substrate level.
That said I will never recommend dry starts. If I could do it all over again I would have planted 1000 individual stems of HC and patiently waited for it to fill in (the right way to do it). Dry start is risky (ESPECIALLY since you and I were inexperienced with planted tanks and tried HC first) and I wasted months of my time waiting for it to fill in.
If you do go ahead with a dry start you will need to have your CO2 dialed in very nicely with good flow. Wait until the HC has taken good hold with decently long roots. It will take at least a month. When you fill the tank immediately dose it with EI levels of ferts. BLAST it with unreasonable amounts of CO2. The plant will need to adapt from having infinite CO2 to as much as you put in the water column. You want to keep the HC trimmed VERY low so it spreads and does not grow over itself. When trimming you have to be very careful. It is really just a tangled mess and it is incredibly easy to uproot chunks of HC. Eventually too much of mine uprooted – and once it comes out it ain’t going back in there.
Also remember that HC loves CO2 – not so much high light. It is NOT a high light plant. Everyone says it needs high light but that is a load of crap. It needs good CO2 and medium light and it will grow just fine.
Get a CO2 setup. DIY CO2 sucks and will only cause your problems in the long run. 5lb tank setup will cost you a few hundred and each 5lb tank will last forever on a tank that small. I BLAST CO2 in my 75 gallon and replace a 5lb every 3 months or so.
Either run a little reactor on a small pump or plumb a reactor to the output of your canister. It’s not that hard to make a cheap little PVC reactor for like $15.
None. Test kits are a waste of time and money IMO. All you need is the API Freshwater Master kit to test Ammonia and Nitrites as your tank cycles. After that it is not worth it. Again – do not chase water parameters. It is not worth it.
With a tank that small I would do at least 50% a week. In the first few months you definitely want to do at least 2x 40% water changes a week. Water changes are VERY IMPORTANT especially as your tank becomes established. Dirty tanks lead to algae problems (especially in the beginning).
Fish should come only once your plants are doing well, CO2 levels are reasonable (not deadly), and you are ready. Fish are the easy part lol. Just don't overstock.
In-line heater with your canister filter on one of those controllers that turn it off if it gets too hot. People whine that heaters malfunction (and they all do eventually) but getting a cheap little controller will turn the heater off when your tank gets too hot. I have a few Hydor inline heaters and they have been great for years.
Powerhead only if you need the flow. Dead spots will create breeding grounds for algae. You will need solid flow to distribute your CO2 and ferts throughout the whole tank. In a 12g this is not hard to do.
I personally take no precautions to get rid of pest snails. Some people bleach dip plants but this may do bad things to HC. Not sure…
With respect to water parameters do not fall into this trap. Deal with the water you have. It is probably fine. If it is absolutely unusable then use RO water (I do not recommend this unless your water is totally unacceptable for fish & plants). Don't chase parameters and don't chase pH. It's not worth it.
Hardscape is up to you! IMO it is worth it in the long run for some nice rocks. You will have them forever and you don’t need many to fill in a 12 gallon… Look at some nice iwugami setups. You will not need more than a few nicely placed stones.
SOME KEY PIECES OF ADVICE:
Be patient. Rushing things only causes problems. It is never worth it. Realistically it will take close to 6 months to get the average tank to where you want it to be.
Don't skimp on hardware. You will only end up buying the nice stuff later on anyways when it becomes obvious that you need it.
Don't let frustration get the best of you. As you long as you put the work in and learn from your mistakes you will have a beautiful tank eventually.
High tech tanks are very difficult. Hopefully you run into less problems than I did with my dry start LOL.