I believe that DDR4/DDR3 SODIMM chips max out at 16GB per dimm
so to get 64GB, you need 4-dimm slots (<- hint... easy to narrow down search, but you'll have very few choices), and specific CPU models
lenovo publishes a technical PDF containing all the specs for their laptops. go browse it and see
https://www.lenovo.com/psref/pdf/tabook.pdf
https://www.lenovo.com/psref/pdf/ThinkPad.pdf
P50 series is heavy though ("workstation" replacement) - starts at 5.6pounds
T460p/E5470 (quad core 35/45W + 32GB) = ~4pounds
Core 2 Duo Penryn 2.53GB
Trust me, it's worth it to upgrade to the latest intel architecture
I had a Dell XPS M1330 laptop (C2D Penryn). Just got a Skylake Dell E5470 laptop(i7 6820HQ 2.7ghz).... worth it, probably 3x faster
(2 cores-> 4 cores = 1x
Penryn->Skylake, maybe 0.5-1x faster clock-for-clcok
faster memory+ssd+HT, another 1x