In your situation I wouldn't get individual tickets but go for the welcome card.
The disadvantage with individual tickets for U-Bahn and bus is that each ticket lasts for two hours after validation, and each journey must be in one direction only. That starts getting tricky when you are going to a whole bunch of different places. For example , are you conceivably going in a direction that could be considered going back in the original direction you came from? That is enough to get you a fine.
With the welcome card you will have transport covered, and discounts on entry to the other places you are thinking of going to.
Brandenburger Tor and Reichstag are right next to one another, so walking distance.
A couple of suggestions:
IMO, Checkpoint Charlie nowadays does not have all that much to see
(though the actual checkpoint building that used to be there, and various other interesting items concerning Berlin such as one of the Berlin airlift planes, an East German Berlin Wall watch tower, and so on, can be seen for free at the Alliierten Museum a short walk from Oskar-Helene Heim station on the U-3 U-Bahn. This is open every day but Monday).
Rather than Checkpoint Charlie, maybe walk south a bit from Brandenburger Tor, past the Holocaust Memorial (BTW, just left from there is the location where Hitler's bunker was located and where he committed suicide. There is NOT a memorial there, and the location is now a car park there).
Continue past the Holocaust Memorial to Potsdamer Platz. There are a few sections of the Berlin Wall there, and pictures of how it looked in the past.
Near Potstdamer Platz is a spy museum, if you are interested, or continue slightly further south, to Niederkirchenerstrasse (the Berlin Wall used to run along here, and Checkpoint Charlie is actually further along this street) and look at the "Topography of Terror", which is located on top of former Gestapo headquarters. It is in the open air, so there is no entry fee, but do be warned it ends up fairly chilling if you read too many of the displays.
Yes, I am very lucky to be living in Berlin. The weight of history here sometimes feels very strong though!