also bear in mind that the 412 S1 is almost identical mechanically to the 411 S4 and S5
I'd agree with Jason with regards to a good body being important because a proper strip down and respray of the aluminium body is expensive if it's done well. But also the interior is also expensive to get re upholstered.
There are one or two issues to look out for in the front suspension, mentioned
here
Also, check the condition of the upper mounting point for the rear shock absorbers.