When you say planetary nebulae, does this mean you want high resolution, deep sky images of small angular size (6-60 arc sec) objects?
This imposes some considerable tracking/seeing constraints....mainly induced by the long (2500-3900mm) focal lengths you'll need to get some image scale.
Adaptive optics will help, but only if you can find a suitably bright guide-star for high frequency guiding.
You would not need CCD acreage, so a modest KAF402 or similar high QE chip would be fine. An SBIG ST7xme and AO8 would certainly fit the bill.
If you don't see any merit in AO, then any cooled (low noise) KAF402 system (Apogee, FLI etc. ) would do.
Hubble's WFPC2 comes to mind
