Thanks Mario and Andy for your opinion and advice.
Maybe servicing the crank is a lot better than buying (without proper inspection) a used crank.

Andy, you state the crank has to be assessed to see whether it is servicable and I learn here (http://tech.nsr-world.com/nsr250/mc21/mc21-crank-specifications.php) which measurements must be within certain tolerances:

1. crankshaft journals measured at A, B, C. I understand measurement at B but the arrow to A points to the LHS crank-arm and C points to the end of the crank where a measurement is probably not possible. Also, I don't understand why the measurements are made with the old and worn bearings sitting in the prism blocks. I would have expected that the measurements are carried out without the bearings.
2. Connecting rod big end axial and lateral play: I assume the conrods cannot be replaced with new ones so this is a combination of wear of the conrod and the crank bearing surface?
3. Conrod small end: Isn't this wear also important?

When you re-build the cranks, which bearings are replaced (i.e. included in the 150 GBP set)?
- Crank RHS, LHS?
- Centre bearing? (What type of bearing is this? ball bearing or needle bearing?)
- Conrod big end bearings?

Is there a need to statically or dynamically balance the crank?

Thanks for answers!
gepe.