Hi Herman
Petrol currently costs 47 Malawi Kwacha per litre which is 0.9996 US Dollar or 0.6880 British Pound.
Car rental is quite expensive but you would need to contact a Car Hire firm to establish the cost. Try contacting
S S Rent-a-Car- Lilongwe Office PO Box 997, Lilongwe Tel: (+265) 741634 Fax: (+265) 741574
or
SS Rent-A-Car - Blantyre Office PO Box 2282, Blantyre Tel: (+265) 621597 or 622836 Fax: (+265) 625074
or any of the others listed on my visitor information page at the following URL:
http://www.africaguide.com/country/malawi/info.htm#hire
As a suggestion always check the small print on your car insurance to make sure you are aware of what you are covered for.
A 4x4 is not necessary in the DRY season, but some of the roads are quite pot-holed and corrugated.
It is safe to travel alone - but it is not adviseable to freecamp anywhere in Malawi or to travel after dark. There are many drunks on the roads and sometimes other drivers tend to drive with their lights off.
Hope this information helps a little. I'm afraid I don't know much about campers....
Regards
Vera