Hi there

Water cooling is safe and most of the those cheaper systems(I mean AIO systems from NZXT or Corsair,Coolermaster etc) work,just proper water cooling that is bit difficult to start as you will need lots of parts and this I wouldn't recommend to "newbie",its not hard to do build water cooled PC if parts are used good

Agree I would go with bigger case,small cases are OK,but I would recommend go with aluminium cases,agree they're heavy,but still you will be not moving case daily,my case without the components weight around 12kg with components I would thought so around 20kg or bit more

I've got CoolerMaster HAF X which is heavy itself,but cooling is just awesome and its very quiet,you shouldn't need any fan controller for this case,if you are using good and quiet CPU cooler,but yes good fan controller with temperature is good investment  

As I've pointed above older i7 is still good CPU,just be sure CPU itself support Hyperthreading,this is great thing when you are rendering and mostly you will gain with this

Really depends on budget,but with yours budget this PC should last you more than previous 

Hope this helps

Thanks,Jura