Most critical parts for SCD-1 & SCD777es are still available. I have both units. You can not beat the build quality of those at their current used price. If you have the money, the SCD-1 is a bit better, but not too much. However, SCD-1 is a statement piece and not just a black box. MCD500 is $6500. Get a SCD-1 and a PS Audio DAC and you will be VERY happy man and you'll still have money to take the wife out. Check out the link below for internal view of both.
http://www.audiokarma.org/forums/showthread.php?t=154689&highlight=inside+scd-1+scd77es
There is no chance that the SCD-1 sounds a bit better than the SCD-777ES, because they are absolutely identical in the parts, which count for sound.
The only differences are the transformer shields, the XLR-symmetrical output board on the SCD-1 (which sounds worse than the asymmetrical ports) and the different looking but equally valuable housing design.
Both units share one service manual, where you can read, how small the differences are.
Spindle motor is especially a problem, if you ship the unit with pulley on the spindle, otherwise it should last very very long ...
An optical pickup costs 139 USD at MCM Electronics.
But there is also small drawback on the two SCDs: reading a CD or SACD needs up to 45 seconds ...
If you write here, that the SCD-1 sounds like a killer machine on SACD and not so well on Red Book, you probably don't know, that red book and SACD run over exactly the same digital filter and D/A chips and analog section as well and you never heard a modified unit, because afterwards the SCD-1/777ES is a
real killer on SACD
and Red Book.
Another hint: There is no other way to come closer to the original master tapes than using a very good SACD player (like a modded SCD-1) and recordings DSD mastered from the original desirably analog material.
IMO e.g. no Tape Project master copy can ever reach this quality, because it includes two additional tape to tape duplication processes.
Try some MFSL Original Master Recordings and you will agree with me.