Since you'll end up doing Intel on LN2, I'd pick an old but nice for testing & learning how to do LN2 OC CPU, like the 2500K or the 2600K.
They have coldbug, so you'll get to learn how to control the temperature during benchmark runs, and it's also a very tweakable platform, so you'll get to learn and get accustomed to all the voltages, clock skews, and the idiosyncrasies of the Intel Core iX platform under LN2.
Plus, it's a very potent CPU, and you'll be able to have fun in both 2D & 3D benchmarks.
If you plan on doing it occasionally you should read and find out about the ideal operating system & tweaks for each benchmark if you want to have comparable scores.
Especially with SuperPi, PiFast, wPrime, 3D Mark2001SE, 3D Mark03 and Aquamark3, bad software tweaking and choices will severely harm your scores and will leave you thinking "why is this guy with lower clocks 3000 points ahead of me ?".