well, if you order one from zzounds.com, youll get a good deal and it will come with a 20 gb hard drive, plenty of space, even a ten gb hard drive will be fine, if it supports a 10 and 20 then it will support a 40, they are ide drives so they are cheap and easy to replace or install, however i wouldnt want to be pulling them in and out all the time. and i dont think you need to worry about any space with a 20gb.
and as far as getting it to the computer, you can use digital, however the best way is to use a cdrw scsci, you can save your tracks as a wave file, as two, or you can save the scene. im not sure how sonar handles scenes yet but im pretty sure you dont have to mix down first to get it to the computer. infact im positive. the dps also can import or export waves at 24bit so it will be able to export any track as a wave, plus the dps supports Connecting a CD-R/RW drive that supports Multi-Media Commands (MMC) enabling you to write audio tracks of the DPS16 directly to a CD-R/RW disc. so basically you can do what ever track you choose and have numerous tracks on a cd. if you export using digital, you just simply should play one track at a time to keep the tracks separate.
playing analog into a computer is basically retarded because if you already recorded it why would you rerecord onto the computer? unless you used the computer as a master recorder, but then you can process the sound digitally anyways with digital transfer. and if you are mastering on a computer you probably do not have external equipment, just plugins, so basically using the cdrw to exchange wav files is the most logical and best way.
if you want a remote recorder this is a good choice, because you can get a clean sound, and if you do any processing on it, it will be 56 bit so no sound loss, then you can keep the audio in the digital domain and transfer it to the computer and master on the computer. hell i master everything on the dps even though i have a good computer setup.