I don't see a song link there, guess something went wrong on your end or mine.
Your laptop would probably be able to do it, but your desktop will be faster... the question is really whether you want to have a dedicated place for your music vs bringing it with you in a smaller version.
 
I haven't used a touch screen but everyone on here that has has been waxing lyrical about them.
 
Definitely check out Omnisphere and/or Komplete. Both of them go on sale from time to time (I got Komplete Ultimate during the last huge Christmas sale). If you can survive until then without them, you'll save yourself some money. I agree with the others that Dimension Pro isn't excellent, but it will work for some in the mix background stuff.

I've yet to see a sale price on Omnisphere.  Lowest I've seen has been $479....I guess down from $499.. maybe that's their sale price?

I would say only you can decide weather certain software sounds better than your keyboard sounds. I have been using a Yamaha S90 since it came out over 10 years ago and I purchased it for the Piano& electric piano sounds, the keyboard was made very well compared to the way digital keyboards are made now. I mean action of the keyboard and less plastic exterior. I have totally replaced all sound from this keyboard with software including the Piano. I think the software today sounds better than the internal sounds of the S90. So, my opinion is that if you find a quality keyboard controller you can do everything you need to, assuming you have your DAW set up right with a decent audio interface so you can plug guitars and mics into it as you mentioned. Sonar X3 and some quality piano & synth software and your all set to go.

The song is    John Murphy - Sunshine (Adagio In D Minor)

I think the included SI-Strings will do pretty decently, perhaps with some stacking.
 
Oh and I thought I had seen Omnisphere for around 380 at some point? I honestly haven't kept track of it, you may well be right.