I believe the Heller kit is generally accurate in outline but somewhat lacking in detail (typical of other Bobcat releases).
The Revell kit, on the other hand, is an absolutely beautiful kit. I only wish they would release it in 1/144th scale.
Is the Heller kit worth building? I think the Revell kit would be a better place to start but since you already have the Heller kit, if it looks good to you then by all means go ahead and build it.
I'm not sure if anyone did after market specifically for the Heller kit. Most aftermarket seems to have been aimed at the Revell kit. If this really is the old Heller Bobcat kit then it probably isn't worth investing in a lot of after market.
If you want to do an Air France Transall, Two Six does a decal sheet (available from Hannants). Recommended kit (wait for it) Revell
(sorry, that's probably not a very satisfactory answer, but in the end only you can decide if a kit is worth building or not)