14 biocube setup/mods

I just bought a 14g cube today used off craigslist. The guy had live sand and water in it I brought it home used the existing sand and bought 15 lbs of.live rock for it and it's currently running. My question is it came with the biocube protein skimmer. Should I run that or just focus on frequent w/c. The other problem is in the first compartment the skimmer will fit bu heater will not fit with it. Also what's the best media for compartment 2 for.mechanicAL filtration on these.. and lastly what should I be looking at on changing on this. I read the stock pumps suck im.going to pick up a mj900 for the return and a small.power head for circulation. Anything else I should think about? This is the first salt tank I have. Wanted to start off small and not get over my head