This may be too late, but I haven't been on for a while.
Go with the external cooler mounted behind the rad for the cheap fix, otherwise get the internal one repaired. High temps aren't going to cause oil to mix with the coolant, but they may cause the cooler to spring a leak if the pressure gets to high, although there should not be much pressure in the cooler anyway. It should be set in the return-to-tank line and effectivley open to atmosphere (in the tank) at the outlet side.