I finally got round to looking at the geometry. Here are my initial thoughts.

On the mesh itself: The tall arm of the frame seems to have some unnecessary polygons. The front wheels would have been better with a denser mesh, as they show fairly obvious flats. The jacking arm does not seem to be positioned correctly.

On the geometry groups: As markschum points out some of the parts  could have been merged, with only material zones used to differentiate between different materials. In general, any part that needs to move MUST be a separate group (or obj with its own unique group name), and any parts that move together as a unit should usually be one geometry group (or obj with one group), with different materials within the group having different material zones. In general the grouping and naming of the groups is very good. Perhaps you could have used capitalization to make the names easier to read "linkarmbottompin" vs "linkArmBottomPin", but this is a very minor point. More annoying is the "new" prefix added to many names. If there is an old part, back it up and remove it, and then remove the prefix from the new part.

All in all the above criticisms are fairly minor points. There is nothing that absolutely must be changed, except the location of the jack handle and jack handle grip.

On the material zones ('usemtl' in the obj): In my view you need four materials (you could have more, but I think four is enough). These could be named (for example) "steel", "paint", "wheel", and "handle". Every part of the geometry should be assigned to one of those material zones.

At a quick look I can find two materials in the engine lift "(null)" and "Material". This does not matter much at the moment when everything is separate obj files, but it will matter a great deal when everything is converted to a figure with one obj file. I think this is the biggest problem with the engine lift at the moment, as it is likely to take the most work to fix.

In spite of what looks like a lot of criticism above, this is really a very nice model, and generally well made. I have seen many models that were complete train wrecks in comparison, and that I would not have tried to rig for a million quid. I think your model should be relatively easy to turn into a working figure.

Things that I suggest you do:

1). Remove any old parts that are no longer needed, eg the (old) 'jackhandle' and (old) 'jackhandlegrip'.

2. (Optional) Rename the geometry groups to remove the prefix "new", and resave the obj files without "new" in the names.

3). Rename the material zones so that each part of the engine lift geometry is assigned to one of the four following material zones, "steel", "paint", "wheel", "handle" (use different names if you prefer).