ok let say first off that i am not a video editing professional, though i have done a little editing myself, i would not even consider myself a
novice since i still get lost in the menus; but i believe i understand the technical aspects of a editing machine.
the best set up for what type of budget you seem to have would be an SSD for the OS and programs, a faster mechanical drive for the scratch/rendering drive; it will bottleneck if you have the scratch disk and program on the same drive (but i think you understand that) and a big platter for all your media storage.
it really would benefit with having one of the 180 Gb SSDs as an OS drive. that will give you enough room w/o having to worry about file managing to conserve space as it would be with a 90 Gb drive. you could get away with a 90 but that will cause quite a few nightmares down the road.
using an SSD as a scratch disk would make rendering extremely fast but pretty much shortens the lifespan and making it practically disposable since the lifespan is measure in disk writes. (you can write to it only so much before it needs replaced) the closest mechanical drive would be a 10,000 rpm veloci-raptor for performance; unless you can afford ~$100 to replace the SSD constantly. so if you can send back/replace the 90/180 SSDs with:
Western Digital VelociRaptor WD3000HLHX 300GB $149.99
even though it will still be only 1/3 the speed of an SSD it will last for years instead of a few months or weeks even.
as far as storage there are several configurations you can have though the general rule of thumb is the more the better; HD content can eat up a lot of room and even a 1Tb drive might end up being a little small; esp if you are storing the finished projects on it. since you have a 1Tb WD black you can use that for the finished projects and the 2 external 1Tb drives to import media/ storage. ideally it might be advantageous to replace the external drives with one of a few 2+Tb interal drives for storage. (this is where some whacky raid configuration can go, if you care to torture yourself like that).
and it would benefit you to look at having a back up strategy in place in case of a disk failure. looking at something like
Acronis® True Image™ Home 2012 so if your SSD fails you can be up and running again without the having to re-install everything and backing up the "projects" disk so you do not experience a financial or time loss having to lose or re-edit the finished projects.
this is by no means a perfect disk configuration. there are RAID0 set ups with 2 WD veloci-raptors for scratch/rendering disks that will substantially decrease for rendering. but i think what i have suggested and a brief explanation why will be a decent "jumping off point" that isn't an excessive start up cost.
hope it helps.