Interesting stuff RB, so as someone with zero knowledge on trains/railway, how many of these new trains have they invested in? Have Merseyrail purchased them, or are they financed/leased etc? How often do they invest in new trains? I'm guessing not very often as I remember the trains they use now years and years ago.
I'm not much clued up on trains or railways, other than from a local history standpoint, but I'll do my best to answer your questions.
Merseyrail have bought the new stock themselves and it will be leased to the operator. The oldest trains in the current fleet are approaching 45 years service, with the younger ones at around 38-39 years. The previous trains, 502s and 503s, also had about 40 to 50 years service, having I think been built between 1936 and 1956.
Based on that, I'd expect the new 777s to have a minimum shelf life of 30 years, as they're based on a very common design in use all over Europe, with spares likely very plentiful. You're probably looking at late 2050s before they even start making plans to replace them.
I think they have 58 trains, with an option for 60 more, as they look to expand the network to Preston, Skelmersdale, Wigan and Wrexham. The new trains have been futureproofed, and can run on overhead wires, and also potentially battery over unelectrified stretches of track.