Help value these watches - looking at purchasing my first vintage Omega

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Hi all,

A close friend of mine has these 2 watches passed on to him by his dad which he is willing to sell. I want to make him an offer and would like to know how much each of these pieces are worth. Here is the information I have;

1. The first watch is a constellation automatic - the black dial has faded and has not been changed in anyway. However the strap is not the original strap

2. The Second watch is a seamster/ De ville manual wound - I am not a 100% certain of the model (neither is my friend who owns the watch). The Dial has been replaced, it used to be a white dial. however the engine/movement works fine. Strap again is not the original strap.

* I haven't opened up either of the watches - since i do not have the expertise to do so and do not want to damage it in any way.
* Both watches do not have box/papers.

Appreciate any advise on valuing these watches please.

Thanks
S