[laxman]

well I have looked into what you are going to do alot as I once thought that it was a project I wanted to do. First off one thing you might want to consider is water cooling the hot side of the peltiers because it will allow them to be alot more effective and will decrease then number of them you need. Although if this is not an option then I would suggest that you use as large of a heat sink as possible. The next thing that I would suggest is that you dont worry about coolling the inside of the case but only the water running through your loop because this will greatly reduce the watt load on your peltiers. To do this just insulate the heck out of all the tubing and and water res. That way you also wont have to worry about making the case air tight or insulated.
On to your questions, you will need around 600 or 700 watts of cooling power to keep your water at sub zero temps. This means that your peltiers will draw around a total of 40 amps at around 16 volts. Thus making a dedicated powersupply needed. next like I said before make the heatsink for the hotside of the peltiers as large as possible as about 900 or so watts will be entering it. Pesonally I would use anti freeze but I have seen other people use other stuff too though. finally frost is the enemy it will break things, so insulate the tubes, the water blocks, and the res. Any other questions? Good luck man you will need it.