I second timb's opinion. There is no real substitute to the old tried-and-true 318...right now. I understand that there is a new 300-series replacement in the wings, but no details yet.
Is the current 300-series the best machine on the market in its class? Probably. But, I have heard that that whole segment of the full-size lawn & garden market has been shrinking. As a result, it seems as though that all of the manufacturers are producing machines that are "dumbed down".
I know two people who run the current 300-series. They would agree with me that the current 300-series can rack up the hours, but they are not the same caliber of tractor that the 318 was. After 1000 hours, the current 300-series feels tired and will start to cost you some money in repairs. One of the guys that I know that has the current 300-series spent over $800 this year alone. The other guy had some engine issues this year, but I don't know specifics. Both guys have around 1000 hours on their 300-series.
The 318 was 25% heavier than the current GX335 (836 lbs. vs. 668 lbs.). Also, the current 300-series uses a vertical-shaft engine and a belt to drive the transmission. The 318 was a shaft-drive tractor.
I remember back when the current 300-series was introduced. I was looking forward to seeing if Deere worked the same magic on them as they did on the 400-series. Boy, was I disappointed!
I voiced my disappointment to a Deere employee at the National Lawn & Garden show in Louisville. I was told that "Harley Davidson used a final belt drive on their motorcycles and the 300-series uses the same philosophy". I wanted to tell him that I would not till my garden or blow snow with a Harley...but I held my tongue. I walked away knowing that I would never buy one.
My father replaced his 332 (diesel version of the 318) with a 455 four-wheel-steer tractor. He felt that the current 300-series would have been a step back, not forward. He is VERY pleased with his 455.
If I were you, I would opt for a good, used 400-series of the previous generation. I like them better than the current X400-series. But, that is just my opinion.