In addition to the F10/141 “sector”, there are hundreds of 1990s vehicles on the preservation scene already- 143s and later F12s and 16s. The convenience of being able to mix with busy present-day traffic must play a part. I wonder how old you can go, before it becomes a nuisance using the motorway, for example? Those 1958 Scania Vabis LV75s seemed well up to the job- 165bhp would power a rigid with a living van on the back, without getting in the way of modern wagons. An early 1960s Leyland, with a P680 in it, would have no difficulty either- I would guess.