The Malawian currency is the Kwacha. There are approx 440 to the  dollar.  A better rate will be obtained at a Forex, can be as much as 20% The black market will give up to 250 to the $. There is no limit to the amount of forex you can take into country, BUT if intending to LEAVE with a substantial amount then it is essential to declare a large amount on entry, otherwise any excess may be confiscated, and you may be prosecuted. This is due to rampant illegal smuggling out of forex, due to the chronic lack of forex in country. You may only take out a max of 3000MK in Malawi currency

Latest rates are available at the National Bank of Malawi website - the exchange page , which is updated nearly every day (FYI - this page can take ages to load, but is usually more up to date than Standard Bank).

There are VISA ATMs in major towns (Lilongwe, Mzuzu, Blantyre). They will only let you withdraw up to around $100 in one transaction, so you may need to make multiple transactions if you need more than that. Bear in mind that ATMs are only available in the larger towns where there is a bank, so if going out into the country make sure you have enough local currency to see you through.