Will these eventually be available as a normal purchase. It seems a bit silly to buy a 325i, and then have it fully modified. I guess it would be hard to sell the 325i's drivers too.
Nope. As far as I know, there's no option to buy a "fully completed" Magnum from Symphones. However, there are a ton of options to put one together for cheap.
Funny to say, but the hardest part to obtain is the cheap Grado headband/rod blocks/gimbals assembly (which I believe is the main reason you can't get them from Symphones). You'd have to pull one off another Grado, or there are some replacement Sony headbands that can fit the cups perfectly. The MDR-V150 or MDR-V6 ones are known to work (~$15 - $20 for the headband).
Rhydon sells the drivers for 120/pair or drivers+aluminum sleeves for 240/pair.
You can buy drivers and then commission someone like Marty or Ohrenholz to build some wooden cups (~150 ish). If you go this route, you'd need a headband (from a Grado or Sony replacement ones), and a cable.
Or you can buy any model Grado, and stick in the drivers+aluminum sleeves. A few people have compared plastic outer cups from the SR60-SR225 Grados, with aluminum outer cups from SR325/MS2, and those don't make much impact on the sound -- so really any Grado can be upgraded.
If you like the Grado coloration (like I do), then these are definitely worth it.