I am also curious about neck dive reported as the ones I have had have been so heavy in the body the neck balanced nicely. The later models of Thunder 1A (version 3!), which had a different Magnabass pickup as opposed to the split P type, were made of a different, lighter wood and, as a result, had some neck dive. The body was the same as the Thunder Jet, I believe, with which it also shared and extra 21st fret! T1Av3 had a different bridge, too. But all the earlier ones with the split P pup I've had (3 over the years) were heavy and nicely balanced.
Either way, you should be able to pick up a nice Thunder 1A for around £150-180. At that price they are massively good value.