The basis is a buchon, yes, but they pretty much "de-buchonized" it as much as it was possible. Red 7 is sofar the most thorough rebuild of a flying 109.
Not entirely sure about the exact aerodynamics, but since the Merlin turns into the opposite direction as the DB 605 (simply because the 605 is basically "upside down
), the fuselage of the Buchon was build to accomodate the torque in the opposite direction as necessary for a DB. That apparently led to the crash of the first D-FMBB. I think the current D-FMBB solves that with trim, but Red 7 is structurally rebuild for a 605. Generally the plane looks (or at least looked in 2005, two crashes ago
) much like the genuine article.
I didn't spot many differences between the genuine "Nesthäkchen" G-4 in Speyer and Red 7 when I looked her over. Even the cockpit was very similar, with original instruments and all, only the ammo counters were replaced by a modern radio/gps. I think there was even a gunsight installed or at least brackets for it.
With Black 2, the whole thing is much more difficult to say as the plane is really a hodgepodge, and even if the used a Buchon as a template they had to replace the whole tail with a late-war wooden assembly. AFAIK the fuselage of Black 2 (D-FDME) is from a czech Avia. Still, Black 2 is my favorite of the EADS 109s, as it is a much more powerful late-war G-10 and it has a historical paint job as "Nose" Müller's Nightfighter with which he scored some of his 30+ RAF bomber kills.
Btw, EADS has a fourth 109, a rebuild Buchon/G2 similar to Red 7, which is apparently not flyable.
Also, some warbird enthusiasts in Freiburg are restoring a russian recovery 109F.