As long as the tubing you get is marked/specced to be oil resistant and such it should be fine... there are tubing in most cooling loops on a car too...
As for putting the rad outside... I use a rad that way...
Up here north it's a great idea... especially in the winter... free subzero coolant...
without a peltier...
I wen't kinda chicken on the fan issue thoo and decided to put the whole rad and fans under our house instead, I don't know the words to describe it in english... but our house is built on a 40-50cm high ventilated ground (common up here becase of the way the ground settles each winter/spring when frozen/thawed, a flat slab would break easily) and this place is free from rain but still somewhat damp... so I isolated well and haven't had any problems for the last 8 months...