I did about a thousand hours in Senecas I & IIs out of Mombasa in the '80s doing probably the exact sort of work you plan to do in Tanzania.
My first thought is that I was flying them 20+ years ago and they weren't new then, so I can't imagine what state they would be in now! and of course these things were not designed to be heavily used in the rough envoirnment that is bush flying in East Africa.
That said it was excellent. The engines were very reliable regularly going to TBO, although as others have said the ones that were single pilot operated were better than the ones that everybody and anybody flew (5Y-CCH for those that remember). The engines had no automatic (modulating) wastegate, the boost was pilot controlled with a "pop-off" valve that opened at 40" of manifold pressure spilling excess boost. The yellow overboost lights came on at 38" to warn you that you were getting close to overboost. We also always ran them at idle for 5' mins post flight to allow the turbine impeller to run down before shutting the engine down. This was to ensure the turbine/impeller shaft and bearings continued to receive oil lubrication.
They all came with the club doors on the left hand side which was very convenient for passenger access, and a club seating is a nice option. There were two fuel configurations and it was very difficult to tell which was which. The standard combination was two inter-connected tanks in ech wing of 25usg each (IIRC). Most of the ones I flew then had the optional long range tanks which consisted of a further small tank between the "main tanks". I can't remember how much exra fuel this gave you.
Thanks to the turbos it was very good out of hot and high bush strips, the slab wing is also very forgiving and allowing for good slow flight characteristics. You have to be careful on landing as it has a tendecy to land very flat so you need to avoid landing on the nose wheel first. I flew the versions with the two blade props and we had to be careful not to chip the blades when operating over stony ground as prop clearance wasn't great. Also be very careful closing the nose baggage door, if it wasn't properly closed it would fly open at liftoff. The prop. wouldn't hit it but falling bags might hit the prop. and of course it is disconcerting!
It was a very good load carrier. ( I am not going to confess how good on a public forum!), with ample baggage space provided pax. bring soft bags that you can squeeze into the relevant baggage compartments!
Our servicing was mainly done in Nairobi (Wilson) by CMC aviation and they had a pretty good bunch of engineers who knew there stuff.
All that said I would tend to go where the others seem to have now gone and save up to buy a Caravan!