Yes, the Xylosynth is basically a Midi controller.
Looking at the info on the mfg web page, I see that the XS does NOT come with any sounds of it's own, but for an extra fee some can be supplied, but they admit that the sounds they can supply aren't that good.
As you say, the computer could be used as a sound source, maybe there are some good digital sounds available that way, but it seems to be a far more complex solution than necessary. Maybe not so bad if you already have the PC, and other bits, and experience of linking them all together.
Maybe you could go that way. But I'm not sure you should go that way immed.
I would suggest you check out some of the modules. Possibilities by Roland, Yamaha, Korg, but there are others. Many possibilities available 2nd hand, prob for less that the mfg charge for their (??) sounds.
Each of the modules will give you basic Marimba, glock, xylophone patches (sounds) to choose from, plus prob other alternative/similar patches, plus the ability to edit the sounds, plus possibilities to use effects and overlays (i.e. playing the same or similar patches using the same notes on different channels, but with maybe slight differences between the two to fatten/modify the sound. Lots of things to experiment with.
BUT, you need to take note of the (fairly sophisticated) midi controllers that the Xylosynth can generate, and you need a tone module that will respond to as many of those controllers as possible. You're paying a LOT for that sophistication, you don't want to waste it by using a unit that disregards the info because it doesn't know about it.
Then, you keep things simple. XS midi out plugs into module midi in. Module audio out plugs into amp in. Yes, there are some possible complications with channel info sent be XS and recognised by module, and how to set sounds on module, but those things can be set and readily activated.