About 20 years ago International brought in a bunch of Century marked Mas 39 rifles that had been converted to 308Win and so marked. They had new barrels installed and were made up of parts from a bin. Real mixmasters. They also had safeties installed on the left side that blocked the trigger from engaging the sear.
I still have the rifle I picked up back then because they included a case of Hirtenberger 7.62 Nato in a combination price at a special offering. The rifle is very good and surprisingly very accurate. It is short/light/handy. I wouldn't feel the least bit undergunned with one. I also have an all matching Mas36 that is minty and in the original chambering. The rifles are identical other than the latter doesn't have a safety. As far as accuracy goes. I would be hard pressed to tell the difference between the two. Both rifles have a fixed front sight and an elevation adjustable rear sight that is just a non adjustable diopter. The sight picture is very similar to a Lee Enfield other than there aren't any sight protectors to help center the front sight.
The original round is just as effective as the 308Win and recoil is about the same. If you are inclined and the rifle you find has a decent bore don't be put off by the negative post. They are not easily come by and many are in good only or worse condition. Like any two piece stock accuracy will suffer if everything isn't tight. Not nearly as fussy as the No1 or No4/5 rifles but still fussy. They don't like bullets (308) that are heavier than 150grains. Mine really likes the pulled 7.62x39 bullets from the cheap corrosive surplus ammo. The bores seem to tolerate the oversized bullets without a hiccup.
OP, a minty 36/51 should bring around $450, depending on whether or not the original sling is with it. Those slings can be very hard to find and run around $50-$75 when you do find them. Also, make sure the grenade launching sights are tight and not bent. Not sure why but I have seen more than a few with bent grenade launching sights.