In brief, then.
There were two gearbox types fitted to the S and SX Sei. One type is identical to the Sei Sporting Box and has conventional, grease filled (hence "dry") inner drive shaft joints. The other uses the same wet type (filled with gearbox oil) shafts as the 899s.
So, to fit one gearbox in place of the other is simple enough, but you need the shafts and cables to match the box in question. Also the clutch cable.
Easy way to tell: on the wet shaft boxes you can engage reverse without pressing down (or lifting up -- it's different for Cinq and Sei, can't remember which is which) the gearbox knob. On the dry shaft boxes you can't.
The box on offer will be a dry shaft Cinq box, so you need the gearbox, both driveshafts, and the gearbox and clutch cables. (Assuming you have the "wet" type box).
As it's a Cinq box, you'll also need the nearside gearbox mount adaptor (may have been fitted to early Seis) to attach the Sei mount to.
None of this is spectacularly difficult.
If you want the reverse detente (again, assuming wet shaft box at the moment) you need the remote box (the bit that includes the gearlever) from a Sei Sporting (or an S or SX with dry shafts), but yours will otherwise work fine.