[CLIVERALLY]

In my opinion based on restoring cars over the years I would of said that unless it is a rare part or indeed a difficult to find one then new is always better. However, the quality of some of the parts from main stream sources has lets say not been brilliant. The cheap Chinese copies of chocolate metal and poor casting often leave a lot to be desired.
If you can restore what you have then do that, if not go for a high quality second hand part, all supposing you can find one.
Stuff like brake calipers are as cheap to get restored as bought new , but chrome and monkey metal (mazark) is more of a challenge.
Stuff like steering boxes, gearboxes, and overdrive units (as long as they are good) are always worth buying secondhand.