Here are some answers for your questions putting in consideration that i have BMW E90 & C180 2013.
first the BMW 320i & C180 works perfectly fine with 92 fuel, and anyone tells you 95 just know nothing.
Second, the new mercedes shape will be launched in egypt in june 2014 which means in a year
third, BMW prices always gets lower then mercedes, and if u check any BMW cars model 2012 vs mercedes 2012 you will see huge different in prices, so dont worry about the re-sale of the mercedes at all.
finally, the 2013 c-180 has some changes than 2012 starting of the front bumper became with LED foglights, extra battery in the trunk, new engine & not Kompressor, and the new 7G-TRONIC transmission & not DSG as one of the people told you in here!!!! mercedes never ever had DSG transmission.
About the passat CC i find its really heavy & weak car on the 1400 cc motor and the re-sale will be extremely bad, also the comparison cant be C180 with 320i, it's either C180 with 316i OR 320i with C200...but please put in mind that the new Mercedes models prices got higher with 20,000 LE to 35,000 LE depending on the model!.
I hope i helped you
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