I don't know if you've got the chance to inspect the rifle. If it were me, I would be reluctant to bid on a modified rifle without the chance to inspect. A few issues come to mind:
i. From the photo it is obvious that the bottom metal has been removed and replaced with a different system. What you can't see is what internal changes they have made to the magazine and trigger arrangements.
ii. It appears the original sight protectors etc have been milled off and replaced by some form of base and ring system. There's no way to tell from the photos whether they made a good job or a misaligned 'bubba' special.
iii. To me the stock looks unfinished, as though it was someone's project rifle that hit the wall resulting in them throwing it together for a quick sale.
iv. Once upon a time, M1917 parts were common as dirt. If something broke there was no difficulty getting replacement parts. Likewise after market options were easily available. I'm not sure that that's the case any more.
On the positive side, the M1917 is a very strong action capable of handling whatever big bore cartridge you want. In a world where the major manufacturers have abandoned magnum length actions, an M17 action isn't a bad option.