[LoBelly]

This is a vast generalisation but...

If a car is actually 90% finished and just needs paint and upholstery then there is no reason why it would not already be registered.

In my experience a modified version of the Pareto Principle may be fairly applied to these sorts of claims - in the kit car sense it would translate as being

80% of the build takes 20% of the time/effort.

My somewhat cynical view of cars sold with such a high % complete is that the owner has realised that it cannot be registered as built and does not have the wherewithal to correct or redo.

Notwithstanding this - if you take a cold look at the parts included that you can/would be willing to use and the value of these against the purchase price you may well find that it is worthwhile for the parts value - to say nothing of the time you spend running around getting them all together.

But beware
Just because it has lights doesn't mean you can use them
Just because it has seats doesn't mean you can use them
...etc

Talk to an engineer first - they will tell you what such a car has to have
then go look at the kit
decide what you think its worth - if you want it at all

have fun

LoBelly