I don't see alot of down side to buying this farm. Close to work, close enough to the family without being "right on top of them". If it isn't what you want you can always sell it down the road. I don't know the area, but land prices here are certainly not getting much cheaper.
If there is not enough room on your dad's farm for both of you in the future, then there is no way for you to farm there unless and until you lose him. Hopefully that is not for MANY years down the road. So it sounds to me that the only way you will be able to farm is to do it on your own. One thing that maybe caused your dad to not endorse the deal is that he sees you going out on your own, doing things different and making your own decisions and his losing "the final say so". so to speak. Not that he is a bad person, but many parents have trouble letting go. He sees this as you being independent of him and he may be afraid of losing his "help" also.
I don't see any big negatives if you want to farm, unless you are going to over extend yourself financially. You can work more and harder while you are this young and be able to enjoy it more a few years down the road.
You never did say if you were married? If so, does your wife have an opinion? If not, now is the time to do it before a family makes it more complicated.