I'll probably write a guide on this because so many people ask me. But to sum it up quickly purchase just cream breads/Vita Bs up until you can't do DT anymore. After that, run with a higher grade (Vita C) and use DS disks to replenish partner DS since it's overall cheaper.
Gizus will be quite costly but I haven't quite worked out the pricing for those yet. But the drops/eggs seem to bring in a good return, so it should give you enough to buy something reasonable to farm with and then some.