that was me like 7 months ago
converted and never looked back. you dont need calcium reactor right away but keep the co2 tank.
you can have live rock and maybe some soft corals with your existing lighting.
look on craigslist for used ones I just got an overflow box for $30 and a drilled 55g for $20
when I started I dropped a ton of cash on equopment only to find the same stuff used and much cheaper
in a 55g I would mix 50 punds of dead base rock wich is cheap, with 30 punds of LR