As others have said, it's not really Southfields, but kinda 'in between' Southfields/Wimbledon.
That specific area has little in the way of shops, cafes, pubs and eateries on the door step, if that's important to you. But within 1.5 miles you can get to Arthur Road (Wimbledon Park small 'high street'), Replingham Road (Southfields slightly bigger 'high street'), and Wimbledon Village (super posh with great pubs, cafes and eateries).
In terms of bigger 'local shopping towns' Wimbledon (main, not village) has lots of shops and cinemas etc. and great transport links, but is a bit tatty these days. The other way you have Putney which is quite nice, particularly down by the river (either side of Putney Bridge).
You would also have good access to Wimbledon Common (nice, country-style walks) and Wimbledon Park (running, tennis, great stuff for kids when they come along).
The whole area is really nice and safe (very, for London). However, to the north-west of you iare quite a few council blocks (mostly low-rise), which sometimes have some 'issues', but I don't see that you would really go through that for much. Though, in London you are never far from council housing anywhere.
For public transport you would probably just walk along Wimbledon Park Road to Southfields Tube, or use the 93 bus route that goes along Wimbeldon Parkside between the transport hubs of Putney or Wimbledon, which both have tubes and also overground trains).
Those implying 'loads of' South Africans are a little bit out of date, though there are still a few around. But you also get plenty of French (as District Line to South Ken is easy and is where the French schools are) and a typically London diverse mix of people from various communities (due in part to to the Mosques in the wider area and also the Buddhapadipa Temple).