Anthony - Parts guns are fine, and probably 90% of the M1 Garands are parts guns, because most of them went through a rebuild where parts got mixed. Even when the rifles are refitted with all the correct parts from Winchester or Springfield, they are still parts rifles. According to Scott Duff in his Red bound M1 Garand publication, "There never was an ALL Winchester Winchester." That is one of the beauties of the M1 Garand, you can mix the parts and still have a legitimate M1 Garand.
If that M1 Garand is original it fits into a special category, and I doubt very much if it currently is, as the Stock Doc said, a Gas Trap, which the Serial number indicates it was originally. A Gas Trap has a fairly large bright Stainless Steel forging at the muzzle that makes it look a lot different than almost every M1 Garand you will ever see. Even Repro Gas Traps (just the Gas Trap) cost a lot more than $900.00.
$900.00 is a lot of money, when you can join a CMP rifle club and buy an M1 Garand (Parts Gun) for $500.00, which will possibly shoot as well if not better than the M1 Garand you are talking about. Now if the guy is selling it for $700.00 with the sling, a bayonet (which I doubt is the "Original" as they were issued separately from the rifle, and never had the rifle serial number on them), the clips, and 500 rounds of .30-06, it might be an OK deal (based upon the cost of the ammo).
The ammo you should shoot in it should not be .30-06 hunting ammo, as that could cause damage to the rifle. It was designed to shoot M2 Ball 147/150 grain bullets, and while 165, 168, and 173 grain bullets work best for competition, you should NEVER use more than 180 grain bullets.
The powder burn rate for most hunting ammo is usually not correct for the M1 Garand. While it works well in a bolt action rifle, most semiauto rifles have timing characteristics for the rifle that are dependent on the burn rate of the powder, to be correct. If you load your own, then use 41 grains of IMR 4895 behind a 168 grain Sierra Match King bullet, and the rifle will shoot well. After you are familiar with the rifle, and get to know others who enjoy shooting the M1 Garand, then you may want to adjust the powder load a little, but don't for now.