Thanks all of you guys for your opinions!

Concerning substrate: I'll go for the quartz sand 0,5-1 mm as I told you (see here http://www.acquaingros.it/edisplay/cat434.htm mid page code AZ053). No further layers of leonardite, laterite etc.. Fertilization will be only flourish tabs under plants.
Concerning water changes: Tom... I'd really like to skip weekly water changes with this tank! I'd rather go for at least 15 days water changes. What do you think? Why you think that weekly changes are required?
Concerning plants: I have some vallisneria left from the previous tank so I think I could re-utilize it. Then, I agree with you and go for bolbitis, java fern and anubias in shadow areas. Swords will be my choice in open areas. What about crypto: nothing at all? I also have another option: nimphea zenkeri. Comments?!?
Concerning CO2: I really would skip it... I know in the long run it'll be far more expensive, but for the moment I'd go for Excel. What would be your recommended dosage to have both beneficial effect on plants and no risk to hurt discus?
Last: I assume that macros (N,P) come from food and fish waste. What about K? Do you think I should add some K2SO4? And micros? also included in food/fish-waste/tap water or some supplement could be necessary (namely Flourish)?

Planned day for new set-up: May, the 6th!!!