The Aztec you are going to fly will probably have an old panel mounted GPS, quite likely a Garmin GPS 100 Many light aircraft in Africa have one of these as the GPS 100 was one of the first units on the market. I presume that you are looking for a modern handheld moving-map style GPS to serve as a back-up.
Probably the best value in a handheld unit for VFR use or as a back-up for IFR is the Lowrance AirMap 500. Street price is about US$425 plus shipping. Check out the on-line reviews, including those by Avionics West and Flying Magazine’s April 2004 issue. The AirMap 500 is amazingly compact, literally shirt pocket size, but has everything you will need for flying in Central Africa or elsewhere. It is extremely easy to use with a manual that was written by a pilot and so is very easy to read and understand. If you decide to get one, make sure it is the International database (part number 112-26) version, and that it has the latest version of the operating software, which I believe is v1.5.0. You can check this on the Lowrance web site. If you need a reliable mail order source, I have had very good service in the past from Marv Golden Pilot Supplies.
The only handheld GPS with a significant performance or feature advantage over the AirMap 500 is the Garmin GPSMAP 296. This has a TAWS-like display of terrain, which could be a life saver in unfamiliar territory in the dark or IMC. However, the 296 will probably set you back over US$1,500 plus shipping. That is more than three times the price of the AirMap 500 and the 296 is also bigger and heavier. My ideal would be for Lowrance to include a TAWS-like display of terrain in a unit the size of the AirMap 500 and at a similar or only slightly increased price. Until this happens the choice is between something like the AirMap 500 or else the 296.
As always it depends on what you want and how much you are willing or able to pay.
You might also want to take into account product quality, support and customer service. Last year I was seeking information on the various handheld GPS products and wrote to Lowrance and Garmin. I received prompt replies from Dave Steensland and Larry Toering at Lowrance. They answered my queries and unasked even sent two follow ups with attachments showing the airspace and ground map details for the part of Africa where I am based. By contrast Garmin has never responded to my letter of 6 November 2003 addressed to Ted Gartner, despite it being sent three times! I also mentioned in that letter that my previous employer early last year had a Garmin GPS in which the memory battery had failed in less than four years. Surely this alone should have attracted a response. I have to conclude that Garmin has both quality control and customer service problems. Lowrance seems to be right on the ball.
Enjoy your flying. The Aztec is a great plane for Africa and driving one around the DRC and adjoining countries will be a wonderful experience.