MYSQL Slow SELECT with multiple joins and ORDER BY
The issue that jumps out at me is the first table in your EXPLAIN:

1   SIMPLE  m3_ const   PRIMARY PRIMARY 4   const   1   Using index; Using temporary; Using filesort 


As you've noticed, if you remove the GROUP BY...ORDER BY..., the query runs much faster. Doing so most likely removes the Using temporary;Using filesort (but I can't know for sure without the CREATE TABLE statements.

I would suspect this is creating the temporary table on disk. See this link on how MySQL uses temporary tables.

First, if you are selecting any BLOB or TEXT columns, this will automatically be an on-disk temporary table. From your list, it looks like r0_.vehicle_offer_description AS vehicle_offer_description6 could fit the bill.

If this is the case, try removing that column and see if there is any noticeable difference.