Hi Jur, and welcome to the HUBB!
We travelled through Africa in 1997, and were never asked for a bank letter or other security to obtain a visa. I would be somewhat nervous about having a letter on my person that indicated I had substantial monies in my bank account. In some countries, that might make you an attractive target for kidnapping!
Instead, you should definitely carry a debit and/or credit card, preferably a couple of cards - one Visa, one Mastercard. Also, carry some travellers cheques with you. That provides sufficient evidence of funds so that obtaining country visas shouldn't be a problem.
As for the letter to go with your medical kit, that is a good idea, especially since you should be carrying your own syringes (needles) in case you need a jab in Africa. Your doctor must write that letter on his letterhead for you, and if you explain why you need it, he will know what to write. It might be useful to have it written in English, if your native language is not widely spoken outside your own country.
Good luck with your trip to Africa, and please keep us posted.
"It matters not what goal you seek
Its secret here reposes:
You've got to dig from week to week
To get Results or Roses."
'One world, two wheels'