Hello, I am a frequent LyX user, and in fact the creator of the Greek locale.
I have never created any LaTeX classes or styles before, but if i got your project correctly, it can all be done with some extra LaTeX commands in the LaTeX preamble of LyX.
In order for the LyX file to reproduce the InDesign output pdf, you will basically need to know:
1. fonts
2. font sizes
3. page sizes and margins
4. indents
All these can be parametrised in LyX preamble by commenting or uncommenting certain commands.
What is important is whether the inDesign pdf will contain graphics and whether you would like their size adjusted to the size of the page. That part, i am not certain i can do, but i could give it a try.
I would appreciate (without you having to accept my offer) if you could give me a sample pdf of what the final output might look like, and let me experiment on the ideas i have. If i manage to produce something acceptable, then you may consider hiring me. The cost, in any case, won't exceed 50euros.
Thanks,
Ulysses