Forget it!
There is no way that SSDs will be cost effective in a small NAS - the network connection is the bottle neck. SSDs are excellent for use in direct access devices, computers, notebooks and such but that speed does not translate over a network.
Even in large data centres SSDs are only used in servers and some NAS boxes that are on a 10 Gigabit fibre network and even then the SSDs are loafing along.
If you are looking for speed you need to upgrade your network to at least 1 Gigabit for everything including the router.