At this point I would recommend washing the boat inside and out with hot water and washing up liquid to remove most of the surface dirt. Then there are two things to assess.
You need to decide what, if anything, is going to need to be replaced or repaired. There are some problems that commonly occur on wooden Mirrors which appear fairly minor, but which require major amounts of work to fix correctly. For example, the side deck batten problem & the (similar, but less common) foredeck batten problem both of which require the decks removed to fix correctly. Another consideration is how well the inside of the buoyancy tanks are sealed. It would be heartbreaking to spend ages restoring a boat, only to find the new paint & varnish does not last because water is getting into the ply via the inside of the buoyancy tanks. Needless to say, the only way to sort this is also to remove the decks, which is a lot of work.
You need to decide what you are going to do regarding paint & varnish. If it's in good condition, it may only be necessary to rub down to produce a matt surface the new paint will adhere to, build up layers where the film has been damaged, and apply a final coat. On the other hand, if the paint is in poor condition, or not adhering to the plywood, or if there are many many layers resulting in a thick film which will not flex (Mirror hulls always flex a bit, and the paint needs to flex as well), or if varnish has been applied to a dirty boat, then the paint or varnish may need to be totally removed.
A bit of a chicken and egg. No point spending hours removing paint from a bottom panel if you are going to replace it. On the other hand, you might need to remove some paint to be able to investigate areas of concern and fully assess if something is going to need to be repaired, or replaced.
A similar assessment should be carried out on the foils (daggerboard & rudder) and the spars.
