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The Celestron First scope is a 76mm tabletop dob and just not worth the money. But my 100mm orion is fantastic, the included eyepieces are actually not complete garbage and the scope responds well to using my Baader Hyperion eyepieces. I first eyeballed Neptune with it, took 30min of star hopping and chart checking but I found it.
Slower lenses are ok but older lenses much less so for astrophotography.
On tripod the faster lens give you more photons therefore more signal in your shots to work with when processing ince you are still limited by exposure duration. On an astromount your have tracking so you can use a slower lens and may need to expose for longer to get the signal you want but since dslr capture is mostly about processing the captures there is little difference between f2.8 and f5.6.
The HUGE difference though is unrelated to aperture, its lens quality. Modern lenses are actually slightly different in construction to provide better image quality with digital sensors. Older film lenses work fine generally but there are technical differences. The biggest issue specific to dslr astrophotography is chromatic aberations and coma effects, basically distortion from the optics in the lens. Older lens suffer noticably from chromatic aberation but modern slow lenses not so much.
However as a photographer lenses are never an identical sharpness throughout their aperture range and rarely (if ever) when at their fastest aperture. To get cleaner rounder stars stopping the aperture down will improve matters but of course you then need to compensate by increasing exposure time and/or iso. Its all a balancing act so best advice is a quality (pricey) lens is basically worth paying for. Avoid the cheap kit lenses and ignore brands, research the lens itself online first (eg Tamron only ever made crap lenses but in recent years they are making some good ones for the price, so buying you need to research the lense model revision itself or you risk getting stuck with dud version). That said I'll repeat dslr astrophotography is mostly about the processing and you can AP with any lense/camera. So if you cheap out on your lense quality there is little you can do to fix that in post. Garbage in gabage out as they say.
Nebulosity needs exposure time, its generally large enough for whatever lense you have. But the built in filters in cameras kills some of it which is why some people mod their cameras and of course ultimately you want a well colled monochrome camera and filter set. As for taking a long time to take enough pics to stack, this is a constant no matter what your gear as it depends on capturing photons to build the picture, only aperture size helps here but still it is slow, even Hubble was exposing for months to gather enough photons so don't bother thinking you can buy something and just press a button and take a photo. I just uploaded M31 to my astrobin gallery (see my sig link) which was taken with a dslr at 200mm on a regular camera tripod only. nothing special needed or used, its just capturing enough frames and processing them to tease out the signal.
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