Laptop hard disks differ in speed (RPM) -the cheapest ones are 4200 RPM, the most expensive 7200 RPM (for comparison, all of the desktop drives are at least 7200 RPM). If you are only playing MIDI files, the slow hard disk will suffice, but if you are going to do sample streaming, I would strongly suggest the fastest drive. Also some of the high-end newer laptops have disks with Serial ATA interface - this is faster than the standard ATA/100 interface you will find in all of the older laptops.
I would choosel a laptop which includes a FireWire (IEEE 1394) port - this will give you flexibility to use a better audio interface. Again, for MIDI-only or 2-channel audio, the USB interface will do fine.
ASUS is a major company, which produces motherboards used by different computer makers. The quality of the case, screen, and general build of the laptop you have to judge for yourself.
Good luck,
Alex
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Regards,
Alex