Using a skimmer does not allow you to go longer between water changes. Skimmers pull out a lot of the nasty proteins that build up in a SW tank -- not the poop, per se, but some of the chemicals the poop give off.
You don't need to go with T5 lighting if you just plan to keep fish only. You could use a shop light if you wish. When you start going with corals, you'll need stronger lighting.
Any heater you use for FW can be used in SW. If you're tank is not drilled or does not have an overflow or is otherwise "reef ready," it's hard to incorporate a sump. In that case, you can use a cannister filter. You can even use a HOB filter, but remember these options have to be cleaned a lot more often. In the SW world, they're referred to as "nitrate factories" but people do use them.
And yes, doing a water change is the same. Just make sure to premix the salt and water prior to the water change. You replace saltwater with saltwater, but if you're just topping off due to evaporation, you add fresh water since the salt does not evaporate.
Just take your time and do your research. You'll find it's not much different than a FW tank.