That is Sucoform 141 or a clone. A semi-rigid coax with PTFE insulation, 71% velocity factor.
It's useable up to 33 GHz, though the SMA connectors won't go so high. Loss is about the same as RG-58. Power handling is pretty high.
BNC work well, make a positive connection, and are simple to crimp. I would suggest that you purchase a pigtail rather than manufacturing one because the SMA connectors are a beast to crimp.
As far as the cables themselves, provided they are still in good shape, both RG-58 used by 10Base2 and RG-8 used by 10Base5 are 50 ohm impedance coax, which is exactly ...
BNC is fine. SMA is probably more common in this application, but BNC is widely available, and easier to swap around than SMA. Since this is receive-only and the pigtail will be short, coax loss isn't a real concern either — you probably want to choose something fairly thin and flexible to minimize the stress on the fragile MCX connector.
I'd go for drilling the hole and having a shorter cable. Go for the thicker cable (RG223 I think). This is a permanent installation, it pays to go the extra mile on this one. Make absolutely sure you have a ground where the antenna enters the house that is also connected to the utility ground via a low-impedance path.
If I can recommend a website for good ...