1. Do I have to use a completely new substrate to get the lush forest or can I just mix in some fluorite?
have to, no. but i highly reccomend it. your plants will simply do much better.
2. Is peat moss under the substrate a really good thing or is more problems then its worth?
i never used peat and never plan to. i have soft water already (kh 3), and now i have aquasoil bringing my kh to 1. and as for ph, CO2 will bring it down, and if you have a kh of 4, you can still safely lower you ph to 6.5. but kh and ph dont matter much for fish, and most plants do fine in high kh and ph.
3. What are some other major thing I need to get?
dry fertz are good (cheap and work).
automating them is also a good option.
4. Should I invest in a RO or tap water just fine? (I have only done fish only and never had a problem)
tap SHOULD be fine. i would test its kh and gh. if the kh is too high, use AS. if the gh is low, use equilibrium. only if it has something freaky like a high NO3  or some NH4, would it be neccissary. it gives you more control, but you should be fine without it.
5.I'm looking to have 2x54 during most of the time and doing a noon burst at 4x54 What are some problems I might run into or should I just stay at 2x54?
for high tech (which you can easily do with pressurized CO2), id run all 4. 4wpg is nice light. and your CO2 can keep up, and you just have to dose enough and you will have good plant growth.