You need both Scavenging/Underworld Trading for mats. Underworld Trading and slicing may both provide schematics but slicing does not give mats.
You need to be able to farm your own mats, especially since Underworld Trading are the rarer mats (Scavenging are the normal mats). So you won't be self sufficient as you will ahve to buy them and buying them will be expensive.
Right, that's a bit of a bummer, but alright. I wasn't sure if UW trading offered anything unique, or was just another way of getting things. I guess I'll go with trading and get slicing and gathering skills on one of my alts.