Your radiator will be brass, not copper. A lot of people forget this.
To be honest, it makes little difference what blocks get stuck in the loop as long as you use appropriate corrosion inhibitor (Purple Ice/Redex or whatever it's called now). Or, if you have too much money, Feser 1 or some other non-conductive fluid.
Copper blocks are, however, the norm and you'll probably struggle to get any other type.
Fitting-wise, think about it. Thread sizes are largely irrelevant as you may buy different components. The important thing is the bit the tubing goes on to. You could have the biggest mix of thread sizes in the world (G1/2, G1/4 or G3/8) and if all your barb ends were for 3/8" tubing, you'd be fine (as long as you had 3/8" interior diameter tubing). Same applies for 1/2" ID tubing, 8mm ID tubing, etc.
Component-wise, I'd get the best you can afford. Watercooling has a nasty habit of starting out cheap and winding up extremely expensive, as it becomes a hobby and - like all hobbies - you try and tweak this and fiddle with that...
Thermochill radiators are among the more expensive and best on the market, althoguh the difference in performance between them and the XSPC RX, Black ICE GTX, TFC whatever is minimal.
Think about your ocmpoment size: can you fit a 120.3 radiator in your case? Or are you going to mount it externally? Are you planning on adding a whole lot of other heat-producing stuff into your loops (like hoter chips, northbridge/GFX/etc.)? If not, then the 120.3 is probably overkill.
Keep reading.
