I had to buy a used auto from 'we finance' like early this year due to no credit establishment. And I have been sucked paying over 200% of the blue book price. And I don't want to keep paying that because financial situation these days is not really good. If I stop paying monthly payment and they repossess the auto, how would it affect my credit and what else is expected to happen?

If they report a repo, and with no positive credit and only a repo your credit report, of course your credit rating would be terrible. Go to FTC.gov and pull a free copy of your report. You can see if they are currently reporting the loan. If so you can be assured the repo would show up, if not they then that means they may not report payment history, but they could report a default or judgement. The same way a cell phone company or utility would do.
Try to negotiate a settlement to pay off the loan, maybe they give you an X% discount for a lump sum payment. Get EVERYTHING in writing. And even then who knows what could happen. Actually I would only pay them through an attorney or CPA, ie they give the attorney the title and you pay the attorney, who signs over the title to you and pays the dealer.

Be careful, simply asking for a settlement can be considered a default, so they could repo the car.
If you default they repo the car will "auction" it for some fraction of the value and then try to collect the deficency balance from you, with interest and fees. Trust me they are the experts in this game, they know exactly what they are doing and have seen any and everything you could think of.