The Winchester 670 was an economy grade rifle that had a good reputation for accuracy (sort of like Remington's 788). The price sounds good, except for the painted stock (?). I would check the gun guides and if the price is correct for one in top condition, I would want some money knocked off for the stock. I also don't think the matte finish is correct for that old a rifle. If it has been extensively refinished, that means no collector value in the future.

If you want it purely as a shooter, then go for it, but you say you would rebarrel it? Add the cost of rebarreling and you could buy a more expensive rifle that is already in .25-06...