Yes, NOS 651 are rather hard to find and the used ones have often rust as they were very exposed to weather : 250 euros is the usual prices for nos lenses, very hard to find.
Also, as the bulb holder was not part of the lamp replacement kit, you need to buy them separetely and, as there is none, the only way to get some is to buy an old pair of Fantastic 651(avg of 150 EUR) . So a 350-400 euros to get the exact set as per mk1 (nos lampes + used bulb holders.
Marchal 656 nos ( not chinese repros like NPD) come out around 150-250 euros per pair. They can even be less : there was; last week, an Ebay seller in Spain who advertised a large set of white 656 at 49 euros per unit, so 98 euros per pair !...https://www.ebay.fr/itm/323904140017. it was a bargain and obviously they have been all sold in a few days.
The benefit of 656 is to come with the bulb holder (as it was glued, it is then part of the replacement lamp...) and to be halogen.
The only difference between a 656 and a 651, is that they added "656" IODE" to the markings on the glass. Beside this all is 100% identical : Shape, size, inside, engravings etc....
So, according to me :
If you want full authenticity, till the bulb holder, then go to 651 (cost 350-400 EUR ), but expect weak light according to modern standards.
If you can compromise on the added "656 IODE" on the 651 glass, go to 656 you will have better light, and save some money (150-250 EUR)
Best