Hi
How do you know its the gearbox thats the problem?. Providing that its not grinding( syncro rings worn) going into 5th then it could be a gearlinkage issue. The easiest way to test would be to disconnect the linkage from the gearbox and try to select 5th directly at the box. If 5th can be selected ok at gearbox, then check linkage for rubbing/distortion/bushings( you will need to reconnect it). 5th gear problems in gearboxes can normally be sorted out without removing gearbox. This is due to the design of most gearboxes (though i dont know the design of the focus gearbox).
Best bet if you cannot do above check, is to bring it to a garage/mechanic and ask to have it looked at.
As to whether to fix it or not , will depend on the cost. If at all possible have it fixed prior to selling.