Old thinkpad expert needed 390X through T21

Been windows shopping for a thinkpad with a backplane speed of 100MHZ.
Laptop hard drive speeds are one of the biggest limiting factors in
performance. I would really rather have a faster 5400 RPM drive than
a slower 4200. I noticed these millenium vintage laptops came with
different size 6.4, 12 and 20 gig drives. When did the Thinkpads
start using faster hard drives and in what sizes?

Thanks in advance!

dsr@Florence.edu