Ok, firstly you could say diving in & around Cape Town could be split into 2 halves. The Atlantic Side & False Bay side.
1 - The Atlantic Side - Its the colder of the two. There are quite a few kelp forests & wrecks to explore here. Most of the diving is shallow, with a lot of the sites being shore entries. There are some deeper sites off shore that are accessed with RIBS
2 - The False Bay side - Here diving is generally warmer and most of the diving being done from Simonstown on down towards Cape point. There are a lot of pleasant shore entry dives available as well as some nice deeper dives which are done off RIBS. If you get the chance try do the wrecks of Smitswinkel Bay.
I know during the winter the weather in the Cape isn't so great with a lot of wind and rain but luckily thats when the best diving is.
As for rental gear & charters I can't say much as I have always used my own gear and only used a charter once whilst diving down there.
You're best bet is to book a dive through a charter and they'll supply u with gear and take you out to a cool dive site.I have found a few websites of charters in the Cape Area for you, maybe e-mail the people and get info on the rental gear.
www.diveaction.co.za , &
www.devillescuba.co.za or look at
http://www.ananzi.co.za/catalog/Sport/WaterSports/ScubaDiving/index.html for some charters in the cape.
By the way I doubt there are any dive charters in South Africa that don't speak English so u don't have to worry about that.
where else you going in South Africa as there are tons of other places to go diving?