Welcome to the forum!
Starting with tap/DI the only reason would be if your water was really bad or you were breeding wild discus or some other soft water fish (discus being the most subjective about "bad water")
I like to buy dry chemicals for dosing so start with Greg Watson and a visit to rexgrigg.com to get going. Basics are KH test kit, bicarb, source of Ca/Mg, macros (N, K, P) and trace mix etc.
I would also recomend something wider than a 55 gal. With a footprint of 48"X12" your options are more limited than say a 75 (48"X18") There are some deep 3' tanks too but be careful with 29's (harder to light.)