I think the "buy cheap, buy twice" only really applies when buying new parts.
i.e. getting the cheapest pump/res combo to go with the cheapest cpu block and the cheapest radiators with the cheapest fans probably isn't a good idea.
Of course buying second hand means everything is more unique and possible not reproducible (in that someoen else couldn't come along and get the exact same parts in exactly the same condition for exactly the same price).
If for example you complete this paying £60 that's great, for you. Would it then be fair to compare this to the cost of a new Silver Arrow and compare the performance? Probably not in my opinion.
When you compare it to the closed loop system what if you get really lucky and someone sells you a H80 for £10 (unlikely I know but...) would you then have to conclude that your custom loop isn't 6 times more effective and as a result that custom watercooling is expensive considering the performance gain you get compared to closed loops?
So not quite sure what it'll prove, but still an interesting project .