Yes your should try the download again to rule out any corruption or other problems with the installer you saved.
As kappy mentioned if the installer you downloaded earlier is in the Applications folder you will need to remove that first so that the MAS store will let you download again.
post back if you still have troubles.
Its a 2008 model, 2x quad core 2.8, QuadroFX, 2x HDs in Raid 1 (1 set 3 TB, 1 set 500 GB). I've tried installing on both RAID arrays and it doesn't work for either.
First off RAID 1 is a mirror and essentially worthless for nearly everyone except if your capturing ET's phone call and a rare hard drive failure at that point would be disastrous for the fate of humanity.
If any problem occurs on the software end, because it's a mirror it gets immediately written to the second drive, now what sort of protection is offered by that? Just about ZERO.
It assumes that hardware is the only point of failure, when sectors on the hard drive also fail, OS X gets corrupted, bad software, human error and even malware all could cause software issues that get immediately written to the other drive!
I can see some purposes where a mirror could be used on data drives, but not to mirror a boot drive, that's just ignorant.
If you have a problem with OS X, say a software issue, then it would be beneficial to have a DELAY before it's written to the other drive.
So what would be more ideal in this case is a automatically updated CLONE of the OS X boot drive, say have it update nightly when you know it's all fine at the end of the day for instance or once a week, or every few days.
So whatever you have now, see if you can Carbon Copy Cloner the OS X boot drive to a blank powered external drive, hold the option/alt key on a wired keyboard and boot from it.
It's it's all ok, then dismantle the boot RAID and install a SSD if you want performance as a boot drive or go back to a single spindle by reformating the hard drive GUID/OS X Ext J and booting from the clone and reverse cloning.
With CCC it also does the Recovery HD, so no worries there, plus it can be schedualed to clone at certain times or when the drive is reconnected, plus remind one ot update the clones, nice piece of software.
Once you back to a single drive, then update to 10.8.
Study RAID concepts and ask questions of seasoned pro's before employing them, join MacGuru's, they know the more serious end of RAID setups.