N52 oil cooler: yes, it's possible
I just talked with someone at AR design and he confirmed that their N54 oil cooler works perfectly with the N52 with no further modification required beyond what is included with the kit. You would have to purchase the package for cars with no oil cooler. It comes with the new thermostat housing that has the distribution hub for the oil cooler. Theoretically, the factory oil cooler kit for the 335 works as well. AR design's can also be used in tandem with the stock cooler for extra cooling.
OK, so why on earth would you need an oil cooler? If you don't do any tracking, it's completely unnecessary. The car does quite well without one even on the track. So if you so track maybe once a year, don't bother. However if you're tracking once or more a month for extended sessions, this is a good idea. I've talked to a few racing teams and they said on hot days it is possible to see 300+ degrees F. I go with Golden Gate Lotus Club and they allow everyone to run intermediate group. So, I can run both advanced and intermediate for lots of track time. This is where the oil cooler comes in handy.
'06 Arctic Metallic 325i | Sport Package | 6MT | Dinan Stage III suspension | Dinan exhaust | 330i manifold swap! Click here!
| Active Autowerke tune | 135i Brake Calipers | Deiselboost caliper brackets | E46 M3 front rotors