Once they seem to get past the 100 - 125 k range there seems to be very little failures so you may be selling it for no reason.
most failures 'seem' to occur in the 60-120k km range. Again I'm sure there are exceptions but that is my feeling around it.
As for some of the other quaetions. 2l TDI seems fairly reliable yes. 3l tdi is very reliable
Arnotur - JUST the parts to open and close that motor(gaskets, bearings, timing components) is in the region of R12k. Pistons from Wossner is around R9k. Add labour and engineering and R45k is right there. Pistons from VW themselves are in the region of R4-6k EACH.