Hello,
I have some experience in this area and would be happy to help you with this. However, a 'lettre de motivation' in French, like a CV in French, should ideally fit on one side of A4 and I think you might struggle to include as much as you would like to. It's always the same, there is always so much more to say than can fit on the page!
I assume you actually want to send the letters and CVs in French, or at least in French as well as in English? If that's the case, I would propose the following:
You write first drafts of your letters in English
I see what's included, edit as I feel best in English.
You check the letters and send comments.
We edit as necessary.
I then translate to French, adding in the correct formal wording at the bottom.
We see how long it is and decide whether the text can realistically fit one one page. If it can't, either we have to shorten the text or you have to 'risk' sending two page letters.
I think that between us we could produce what you need.
What you don't want to do, in my opinion, is get someone to write your letters in English and then have them translated to French. I think that what you need to do is to develop the two in parallel, so that the letters in both languages 'fit" the French ideal.
I'm quoting $100 because it would take some time sending letters back and forth and making revisions etc. If it took less time I would reduce the fee accordingly.
Please let me know if you feel I can help.
Kind regards
Heather Tombs