well im still pretty new doha, just over a month since i landed so i will try my best to answer some of your questions...

car prices: huge difference in price compared to SA, they are cheaper over here. check out http://www.qatarliving.com/node/1940 for prices of new cars...

cant say i've come accross any diesel cars over here... price of petrol is realllllly cheap. can fill up a tank for about R110... (always a great experience to go and fill up :-p)

price of groceries, check out: http://www.qatarliving.com/node/1876 for prices in Doha, the exhange rate is currently at 1.86 rands for every riyal...
prices tend to vary compared to SA depending on product types, fruit & veg are a more expensive than SA... other things such as milk/bread are about the same... and certain items may be cheaper like meat etc.... bought some south african oranges the other day, was really good :-)

there are currently 2000 south africans in doha (so says the embassy), the city is not very big so you tend to bump into some south africans once in a while... you will hear the odd couple speaking afrikaans and you'll see a few cars with the SA flag stuck on the back window. i think there are some south african groups that meet up, they have a ladies coffee morning event and a few other things, havent attended anything yet so cant really tell you about that.

hope this helps... let me know if you need any other info...