An oil cooler will benifit any modification that will cause the engine to heat up oil hotter then a standard engine will, or for any engine that will be used at high load for long durations. A turbo increases engine heat dramatically so it's worth getting one, preferably with a thermostat so it doesn't run the oil too cool.
A sandwich plate between the block and oil filter might be the best option, you don't need to run an oil cooler for that. Or the cheaper alternative is a tee on the oil pressure switch. The only downside with this method is it will pump the oil before the filter, so as debris builds up in the sump, it could potentially send this to your turbo and wear bearings quicker. I ran that set up for a few years and didn't have any issues though. But from a sandwich plate post filter feed is probably best.