I recently acquired several 00 gauge loco chassis and some 5 mixed bogies from ebay.
Two were Bachmann 37's ( one with a missing bogie)
One Bachmann diesel 1-Co Co-1 chassis with bogies
One Co Co diesel chassis
One unidentified peak or Class 40 chassis.
A little research soon identified the co co as a Heljan 47 and two of the spare bogies looked like they belonged to the 47 but no bogie clips or drive shaft.
One of the bogies looked like it belonged to the Bachmann 37.
All seemed to have motors , but only the co co had a PCB.
Anyway I bid and won the lot for the sum of £102.
I enjoyed opening the box and soon found my research was correct.
I found Bachmann class 45 drive shaft fit the Heljan 47, but I did have to fabricate some bogie gear tower clips as Heljan don't have spares.
So after a little soldering I now have 2 working class 37's , a working Heljan 47( no mazak rot either)
and a working class 45 chassis.
The fifth one is an earlier style Bachmann 45 ( central motor) and the drive down through the years works well but it's missing bogie frames and wheels.
I had 2 x Vi train class 37 bodyshells( £8 each from a show) and will look out for a Heljan 47 and2 x Bachmann peak bodyshells at future shows.
The 47 had a pcb and is now DCC fitted, others were DC and I'll fit s decoder sockets to them all in due course .
So at around £25 each and if you add say £25/£30 for the two missing bodyshells that's still around £50 per loco.
Of course, having a decent selection of spares does help, and the ability to make things like the bogie clips. but there are still bargains but they tend to be things which most modelled would put in the ' too much trouble category.