Unless they are out to take advantage of you, none here will respond to the following:
Which is the best site to get a thesis of 100,000 words professionally being written down?
If any do, it is in your best interest to ignore them. What you need to know is that many here have a vested interest in promoting certain sites/writers. Generally speaking, the ones who do so are the low-quality writers/site owners. They need to trawl this forum in search of clients as they hardly ever get repeat business or referrals. A great many of them pose as satisfied students/customers. Hence ... it is in your interest to ignore any/all response to your above request.
Can I just my name on the thesis which the writer will write for me?
Unwise.
Will the Univ be able to catch me for completing a thesis in such a manner -cheating/plagiarism/fraud?
Supposing that the thesis you receive is, indeed, original and not copy/pasted from all over, you will be safe as far as the originality report is concerned. However, do supervisors simply rely on turnitin reports to determine plagiarism? No. You could get caught during discussions with your supervisor and during your viva (when it becomes evident that you really do not know the study). Another thing to think about - as far as a significant number of sites are concerned, the chances of your receiving 100,000 plagiarism-free words are almost nil.
I am also interested in publishing the Chapters as Articles in reputed journals? So after publishing what if the writer claims it to be his own work?
The writer cannot do that or, at least, should not. You should not be doing that either.