Well starting a SW is not the hard part - maintaining one is where it gets tricky. Are you wanting to only get perculas, or perhaps add more livestock later on? Plan it out because they get territorial. Also if you want perc's you need to consider some anemonies too, since they make certain types their homes. Different clowns use different anemonies, but perc's (Nemo's) tend to prefer bubble-tips and a few others.
As far as the FW sump goes, you will need to toss out some of that media, the bio-balls are fine but will need to be rinsed and cleaned, as with the rest of your unit. Algae, bacteria, etc. from a fresh water set up is nothing you want introduced into your SW set up, and even though most would die off, it still will shock your params and make cycling a little more frustrating than it already is sometimes.
If you intend on having live rock, sand, inverts and corals (which you will need some), then you also have to consider a protein skimmer, and the lighting you are going to use in the tank. Also, do you have a picture of your sump? And also of your tank? How big is it - 55gal? If so you can get a small school of clowns if those are the only ones you wish to add. They do much better in schools anyhow as they establish pecking order and pairs.
Think about what you want now, and potentially in the future. What you are working with now, and want to purchase/upgrade too, etc. and then fill us in.