Sticky aperture and cracked glass on a lens for which parts are tough to find would be a dealbreaker for me. Imagine if DIY cleaning doesn't fix the aperture, you send it to KEH, and they find it needs some part that is no longer available. The glass crack looks like it's all the way through, so shouldn't expand out into the middle of the glass, but glass chips could be floating around inside contributing to the aperture issue.
I would only take the risk keeping that lens if the seller was willing to drop the price from $2600 to the $500 range. That makes it a good gamble on your part. Note that after you try cleaning the aperture, the seller can reasonably refuse to take a return, because they can't tell whether you made things worse.
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As an alternative, consider a 6" Schmidt-Cassegrain telescope for an inexpensive 1500mm focal length f/10 lens. It will be slow manual focus but at that distance it should be relatively easy to keep in focus. Your location is "Virginia" so I assume you'll be photographing Wallops launches; those are medium size rockets and 1500mm probably won't be too much focal length. f/10 seems slow for night launches but the bright glow of rocket exhaust will be a limiting factor regardless of aperture.
If you decide to go the telescope route, I suggest ordering from High Point Scientific because of their outstanding technical support. They can help match a t-ring for your camera, a t-to-SCT or t-to-eyepiece adapter for the telescope, and a tripod adapter for your existing tripod (if heavy duty) or a new tripod.
Celestron C6-A SCT | Purchase Celestron C6-A SCT Aluminum Optical Tube, Celestron Optical Tubes Online at High Point Scientific