Mercurial Longsword is from Sword&Fist 3.0 edittion book, I do not know any net source provide info from it but the book does really exchist and so does the Mercurial Longsword.
Anycase it's stats are what I have listed;
Weight: I recall it was 4lb but I can re-check
Price: I do not recall exact one but if Mercurial Greatsword costs around 600gp (non-masterwork) then it would be around half of that value (300gp).
Type: Slashing and it is exotic one handed weapon and non-profient user suffers -6 to use it isntead -4 because it is more odd balanced sword and also heavy to use one handd than bastard sword.
Description: Inside the blade of Mercurial Longsword is placed quicksilver which goes to weapon's tip when the weapon is swinged. So the sword most damage comes from that and blade is hollow and re-counter followness the blade is made wider and bigger to avoid reduction of toughness.
Anycase I am okay that if I do not start whit masterwork gear, but you said, as I recall, that we can start whit +1 weapon, armor, minor magic item etc if we go trough academy right? If so, I would replace that bonus whit that all my gear is masterwork (or do I confuse this rule from other game). Anycase I would ask get the Mercurial Longsword as background item, I cannot obviously buy and start whit it but I cna buy my armors and have about 20gp left (adn thsi means also byebey for my bow).