you cant get all those answers by searching google...actually there is a guide on this forum somewhere...so do a little research before postin, cuz people get sick of people answering the same Q's over and over...
now the problem with this is that a) if you don't know half of these things without asking you probably shouldn't be messing with water cooling and b) if you are only prepared to get the cheapest parts you shouldn't be messing with watercooling. water cooling is for the true enthusiast and buying cheap can cost you big in the long run.
on topic: probably the aquagate mini r180 then the TT bigwater SE would be cheapest but i really wouldn't reccomend them.
if the computer has a shutdown temp then it should turn off when it overheats in the case of running out of coolant. you should use a transparent reservoir and have flow meters/level meters so you would know if you were running out of coolant
tap water can be used but is not advisable due to its impurities allowing for algal growth and corrosion within the water loop
you don't just run out of coolant, that's why you have a reservoir or T-line and top those off from time to time although you really only have to do that when you first bleed the system
I have never needed to top up my water cooling (TT bigwater SE - as I have said many times, I got it because I was bored and it looked kewl.) (custom watercooling in the UK also costs twice as much as it does in the US as well. - my defence over.)
The water level will stay constant unless if there is a leak but then you would have bigger problems than the CPU overheating.
Tapwater is fine if boiled off first and you replace the res and tubing ever 3 months.
say, does the amount of tube you use have an effect on the cooling, cos i was thinkin it would look cool to make lots of tube sized hoes in the case, and then the tude could come out the case and go in somewhere else, and look all spacey
This will reduace the speed of the water having to go the longer distance and round more corners, so any gain of the heat would be greatly lost due to the slower speed of the liquid.