JED70 has some good suggestions^
Larger ssd's are a bit faster, a 240gb ssd is good.
I meant my comment on the Samsung pro to suggest that the plain 840 would be equally good.
If there is a price difference, there is no real need to spend more on the pro version. You will never see the benefits. A SSD will be obsolete long before endurance becomes a problem.
If this is a new build, I sugest deferring on the hard drive and build using the ssd only. It is easy to add a hard drive for storage later if you need to.
SSD for cache does not make sense to me. If a file is heavily used, put it on the ssd in the first place. A cache is useful only in very specific scenarios. If, for example, there is a set of read only files that you will use this week that takes 64gb,and next week an entirely different 64gb set will be used then a 64gb cache can be good.
But if you use 74gb it will not work well, if you are updating, it takes double updates, and if you are constantly accessing different parts of the hard drive, the cache will not do much good.