Its recommended to find the same type of disk. This limits the complexities. The RAID controller might not like some oddball disk in there. If you cannot source this disk, pick the most similiar disk possible. If you have a Seagate ES400 60gig and you can't find it, then buy the 80 or 100gb model. In a pinch, most disks will work, but why take that chance unless you absolutely have to?
SSD's wont do you any good. First off, the array is only as fast as its slowest disk. Secondly, SSD's are a reliability nightmare and your RAID controller may just reject it. I could see the argument to build a fresh array using a SLC (not MLC!!) SSD with a RAID card known to work well with SSD and which, ideally, can do TRIM on the array.
I'd also google your RAID card model + name of proposed drive and see what comes up.
Lastly, you need an enterprise grade drive to do RAID. Don't just buy whats on sale at newegg. Consumer drives aren't tested or designed to do RAID and can cause issues (going to sleep, overheat, greatly slow down the array, corrupt data).