Hi,
Welcome to the hobby =)
I'm new to the hobby so I'll share some of the things which I've learned along the way..
Confirm with the previous owner if the live rock are truly "live".. You could save some money on the filtration if the live rock to water volume ratio is decent (1kg/litre), I believe a canister filter is not necessary considering the low bioload of the tank (a couple of crabs + clownfish)
I'd suggest getting enough salt mix for a water changes, 5-10% twice a week to keep the pollutants in check..
If you have a habit of overfeeding your fish, start looking around for a protein skimmer.
I assume budget is a concern, please ask around the forum for a 2nd hand skimmer.
That should tide you around while you learn more about skimmers. (There are cheap MIC brands in the LFS which cost a fraction of the normal skimmers and work pretty well =D)
I am a fan of high water volume turnover, so if you can, get a koralia rated for your tank so that you can reduce the dead spots in the tank.. Better circulation = better inhabitants
Remember to plan for the area which you want the tank to be placed, then get a nice stand. Sumps are a nice thing to have, but not exactly necessary at this point in time. You can add a sump to your set up at a later date with external overflows.
Hope it helps =)