MARANTZ MODEL 10 TUBE AMP/ TUNER AND MARANTZ 7 TUBE PREAMP

Also, of course, missing or non-original parts can adversely affect value, in some cases even if the non-original parts restore or improve performance. That can present a dilemma when it comes to classic pieces such as these.

Yes- back when the Japanese were scouring the country looking for equipment like this (1980s and 1990s), it was really important to them that the equipment was in original stock condition, despite that meaning that it was essentially unusable! 
So a restoration should be done that makes it really hard to tell it happened.

The owner told me that amp and preamp was used from time to time and they stopped using it when they removed their ceiling speakers recently.

Sounds to me like a bit of luck that the stuff is still running in that case. It really is a good idea to get this stuff serviced. You'll be hating yourself if a power transformer fails otherwise!
Its not that hard for any good technician. I've rebuilt several model 7s and 10Bs.

Any older tube gear that you expect to press into service will need the filter capacitors in the power supplies replaced at the bare minimum.

Have they been refurbished? If not, I would only plug them in long enough to see that they light up; any longer than that and the power transformers are at risk due to failing filter capacitors in the power supplies.
If they have been refurbished, what sort of warranty do you have? The power transformers will likely have to be re-manufactured should one fail, which is not cheap.