'bout a '75.
No, it's a trials bike (you know standing up riding over rocks, logs etc and trying not to put your foot down
).
The 172 is a good bike, but $2,000 is outragous. $1,000 if in good (running) condition.
The 172 has 21"/18' wheels which the 123 did not. Avoid the 123 for this reason (see earlier thread).
In its day the 172 was considered a bit of a giant killer with guys like Brian Hutchinson & John Hemingway regularly challenging Martin Lampkin, Malcolm Rathmell et al for top spots in the UK championship.