I have a discus tank with swords and vallis and not a lot else apart from a tigerlotus as the discus need the floor space and are messy little buggers when eating
As said, get the biggest you can afford, I buy mine from devotedley discus - mail order, they are much better quality than any you will find in a lfs as are all the main discus dealers/importers in the uk.
Once they start spawning theres no stopping them, raising the tds or the ph does nothing - I have tried and was told

you dont want them to spawn if possible as they fight. I have five 12cm, one 18cm and two 10cm in my tank and I get three spawing about once a week - in a community tank you will get no fry, the eggs will last around 36-48 hours and then be fungussed or eaten.
Theres nothing wrong with using c02 in a discus tank as long as you are careful, having said that, my ph has gone as low as 6.2 and a kh of 4 before now and not had a problem.
I would recomend searching for a specialist discus site and doing a lot of research first send me a pm if you want any sites/forums worth looking at for them