Well it comes down to money. As soon as you say 1U the price jumps up and these items are very noisy from the small fans turning fast all the time trying to keep such a compact unit cool. My suggestions are to first figure out if you want an entire computer or if you want to piece it together. Next look for a case you like. You need to know if you need 4 drive bays, 6, 9.... Whatever you need now and what you think in the future. Do you know how much storage you are looking for? Here is a great link for a RAIDZ pool size calculator
http://www.servethehome.com/raid-calculator/ just fill in the information and see what you get. If your data is considered critical, go for RAIDZ2 and if it's mission critical then go for RAIDZ3. Do not use "spare" drives unless you are already using a RAIDZ3 system, it just doesn't make sense. As for the drives well if this system will be having a lot of I/O (lots of users) then you might want high speed enterprise NAS drives, if its home use then the WD Red NAS drives are a good choice, I have 5 of them, no complaints.
RAM, buy a server/MB that can handle at least 16GB RAM (my personal opinion) and if this is for high I/O then you will want probably 128GB RAM if you can afford it. RAM is speed up to a certain point.
Now if this is for a home system and you don't need a 1U rack, fiind any other case that is large enough to hold the number of drives you want. I say last week someone here was building a system, took a case and some Hot Swap drive bays, added those to the case and was able to achieve 9 quick release hot swap bays accessible from the case front. Right now I am using an old computer case, it can hold 10 drives if needed but there is no hot swap or quick swap. I'm looking for another option but not ready to purchase yet. I need one from drive so I can build a RAIDZ2 system. That's $115 bucks I don't have right this second.
As for a MB/CPU, we would need to know what you expect out of the NAS. Ultra High-Speed means $$$$ but good speed like 100MB/Sec write, 80MB/read, that is not expensive.
Run thought these forums and see what others are building. FreeNAS should run on most systems providing you don't start adding raid cards, things can get tricky there and if you want that later, you could add it later.