new RO/DI system... additives??

Hello. I have a new sal****er tank with 2 oscellaris clowns in it.
Ammonia
and nitrite has been reading 0 in it for over a week now. They are in
a 55 gallon tank w/ 20 lbs live rock (bought from a 3 year old
established tank) and 60 lbs live sand.
Nitrates are reading about 40 (using Mardel paper strip test). My
tank is going to be fish only with a little live rock... I plan on
having 2 clowns, 2 cleaner shrimp, 1 small blue tang,and I am still
unsure about what I should have as a cleanup crew... any suggestions?

I am about to buy a RO or RO/DI system. Should I get the RO system
only? or
the RO/DI one? I've been browsing the newsgroups for awhile and
there's a
lot of conflicting info on what additives to add to the RO water...
my tap water ph is around 8, but I'm assuming that RO water will be
more around ph 7... do I need to add something to bring the ph up to
8-8.4? is it different for water changes and top-offs? should i use
DI water for both? btw, I
currently use instant ocean sea salt.

Thanks!