I will start with 20307, I made this using an A1 Models conversion kit and a Lima model as the base, this was before Shawplan had done their finer etched conversion. I would have used the Shawplan parts had they been available. This was probably about ten years ago. It is very lightly weathered.
20303 was purchased, by a friend from Ebay recently. It was purchased with a different number, I changed it before weathering. It also had screw couplings but as it was to run as a second loco it had to have tension locks at both ends. The base loco is a Bachmann and it is assumed the conversion was using Shawplan parts. I would say the work was carried out to a good standard. The paint however was very glossy either by design or accident, so a dulling down coat of varnish was applied before quite a heavy weathering.
Conclusion, of course the Bachmann/Shawplan is the best way to go, but I still have a soft spot for the Lima Class 20. I actually recently purchased a further A1 DRS conversion for class 20 which I hope to do soon.