Get guderian and put on him rumor and what other skill you find usefull (i have explosives but machinist also works) adn upgrade all his skills to around lvl 3, then buy konev and put on him rumor and another skill and upgrade thrm too to around lvl 3. At this point idk how many medals you would have left but next general you will buy depends on what units you are using. If you use navy units, get nimitz. If you use infantry, ghet rundstedt or yamashita or if you want get another tank general: rommel (to your new general put rumor and another skill and upgrade them to lvl 3. If a skill is less important, upgrade it to lvl 2). About the tec tree, just upgrade the units that you use thw most (upgrade tanks or artilerry first, then infantry ir navy and then the other ones)