CASE 70XT Touble Starting with new Exide Battery and Newly Rebuilt Starter

I have a 2003 Case 70XT that has had trouble starting especially below 30 degrees. Wont start at all below 10 degrees unless it is plugged in for at least 4 hours. This has been like this since I bought it new. Just the other day, I purchased an Exide AGM battery with 850 CCA @ 0 Degrees (exceeds OEM battery) and still had trouble turning over. I tested with multimeter and had the following - 12.5V sitting cold then drop to 8V when turning over. I then had the starter rebuilt and the only thing that the rebuilder said was the contacts looked a little burnt but still okay and the bearings were a little noisy. He made it like new again and tested it then I put it back in the machine only to find the same drop in voltage. I checked all the grounds back to the battery and all have zero resistance. A mechanic told me that I should consider replacing the starter with a brand new OEM but that is $$$ and not sure if the starter is the issue.

Can someone tell me if there is another way I should be testing to see where the issue is coming from? Could it be a battery issue? Again this has been happening since new and this is the 3rd battery the machine has seen. The 2nd battery was a Duralast with 1000 CCA @ 32 degrees and it lasted 5 years but again had trouble starting in the winter just like the Exide. Not sure if this is just the nature of the equipment or if there is an underlying issue. I was told that I should not see anymore than a 2 volt drop when it is turning over no matter the temperature.... Any help would be much appreciated.

Thanks,
Louie

CASE 70XT Touble Starting with new Exide Battery and Newly Rebuilt Starter