Talhoffer was 15th Century, born after the period in which the game is set, (born 1410-1415) so I’d stay away from his fechtbuch in this case.
The Fiore manuscript also dates to a couple years after the setting of Kingdom Come (oldest known copy dates to 1409), however Fiore himself WAS active during this time period, so even if he wrote Flower of Battle afterwards, the art as he was teaching it was certainly being practice at the time.
Given the setting, though, I would argue in favor of the German style. Ringeck’s manuscript is somewhat later in the century, however Ringeck himself was either a direct student of Liechtenauer, or a student of a student. Additionally, his fechtbuch is apparently a more or less direct translation of the Liechtenauer’s coded verse, rather than adding anything new. And it’s also the most complete gloss of the Master’s work.