I think an MS 905 is nicer , faster,  has more features , and an internal battery but will put you  a little over your  $1500 budget. A DS708 is still a capable tool that you can piggyback  cheap 12 v lith ion battery power pack  to for about $25 , but from what I believe support posted awhile ago ,the DS 708 is going to  be discontinued in 2017 and only be supported with updates until 2020.Since the DS708 is no longer the flagship scanner, it makes me wonder if  updating  it may actually downgrade some of the OE functions it still had a year ago and more of engineering focus has shifted to sorting out the newer platform's problems than the older ones
   A  DS 708's  hard drive is the contents of the   4GB SD card that they come shipped with out of the box (unless the newer ones come with a larger 8GB SD card)
All the  updated software will likely no longer fit on a 4GB card , and if you were to try to fit all available car makes and updates , then something isn't going to fit , stuff will be missing or card  might get corrupted. Kinda the same reason why Italy is shaped like a boot cause  all that .... ain't gonna fit in a loafer.
If you do keep the DS 708 , the first thing I would do IS NOT to put the Virgin  4GB software card in the unit, and power up the unit. Take that card and  just create a back up image of the card on your PC then copy the image to a larger  16GB Sandisk brand class 3 or class 4 SD card . I wouldn't ever install the original 4GB software card .Just keep that original virgin 4GB SD software card the way it came out of box  in a safe place. After verifying all the files transferred from the original 4Gb SD card to the 16GB SD card (file contents totals between the 2 cards should be the same size if everything transferred), then install the DS 708 software suite  on your PC from the CD.Put the 16GB SD card in your DS 708 and power on the unit for the first time , wait for it to boot , click Setup/help, click About. Write down  everything displayed including Serial Number and Register Password and store in a safe place.