As Gordon correctly says, there are several types of astrophotography. If you have read all
this thread, (and the links he has provided), you should now be able to phrase a question better than you did. The earlier one is akin to buying a toy rocket & asking how you get it into orbit
You say you have a 76/700 'scope - I assume you mean something like the National Geographic reflector on an equatorial mount ? That's not really a good one to use for astrophotography for a lot of reasons (regardless of what they may tell you - none of the toy-shop 'scopes are much use, to be brutally honest). The big one being - it's not motorised; add onto that the fact it's an f/9 scope, and you have quite limited time before the object has moved enough distance to make the image worthless (f/9 means you have a slow 'scope - same as in photography).
Using the more powerful lens (the 9mm), you'll get around x144 magnification; the best you can get (with a different lens) will be around x150 (the rule-of-thumb is x50 per inch diameter you have - yours is 3" so that's x150; mine's 10" so that's x500 - all in perfect conditions too, which rarely happen), so don't waste money on smaller lenses (you'll see the degradation when you use the Barlow with the 9mm to get to about x288). That (and a method of holding the camera to the eyepiece) should rule out #2 & 3.
Prime focus likely should
be the way to go; that means you need to investigate where to get & how to deploy a webcam adapter to the Pi Camera (I don't see any advantage in the NoIR compared to the ordinary one); adapters do exist to replace the Pi camera's lens to permit such an adapter to be attached. Call it about £50 for the parts, but you may want to buy the v2 camera, as this has more pixels ... Any image you can get out of your 3" 'scope will look better (once processed) with the v2 camera compared to the v1 camera ... and yes, you will
need to process it.