How to pick a scope and probe to measure a 12.5Gbps signaling frequency?
To begin, 12.5 Gbps signaling link has the main frequency of 6.25 GHz. The unit interval is 80ps, true, but the period of signal is 160ps. 

Now, all modern designs of communication links are more concerned with reducing parasitic emissions, so the signals are designed to have as low edge rate as possible, just enough to deliver a decent eye opening. So 1/10th of the signal rate is plenty, typically the edge transition makes about 1/3 of the Unit Interval (or about 25ps for 12,5 Gbps link). And silicon designers must work really hard to get edges faster than 20ps. So a 40GHz real-time scope should be fine. 

If a non-monotonicty of egdes are still of concern, sampling scopes can do a better job, see this presentation from Agilent/Keysight. This was 10 years ago. You can find a used Agilent 86100C for just $20,000 - $30,000.