I'm very pleased with how this thread has gone and want to thank all of you that have given me things to think about.
So far I've concluded that my primary consideration should be the lens system and that as interesting as micro four thirds systems are, the lenses I want to go with are the Canon ones. The availability of 2nd hand Canon L lenses are definitely a part of that. I really want a L series 70-200 for the motorsport photography and the f4 IS or the f4 IS II would be my pick for weight/price/image quality. I think I'll run into problems at the wider end and will also need a 24-70, maybe a 2nd hand 24-70 f2.8.
So, what body to connect those lenses to?
I'm thinking that a 2nd hand 7D MkII isn't completely what I want, but if one comes up on TM at a good price then it has to be worth considering, I can always buy a new camera body in a few years and use existing lenses with it.
If I buy new then FF cameras have an appeal. Not really essential for sports photography, but they do offer benefits for other types of photography. If I'm lugging around heavy L glass, then why not a decent FF body too?
I haven't ruled out Sony Alpha A7 series cameras, I would probably go with the top quality Metabones EF to E mount adaptor, not cheap but hardly the dearest piece of equipment I'm looking at!
The Canon 5D Mk III 2nd hand is an option, quite a few improvements on the MkII and even 6fps is probably fine for sports shooting. It is a solid performer and would give good results for any type of photography I would do.
The M6 MkII with EF lens adaptor would be a temping option, but not dust/weather sealed means that I'll probably rule this one out.
The 6D MkII with 6.5fps is a FF D-SLR worth considering I think.
The EOS-R with EF adaptor could be worth considering, but it seems like a MkII is sorely needed.