I don't know where you are from, but why would the bike be restricted? It seems any bike that has been modified can be unmodified. If it's a 600cc that is restricted because of your age or something then buy it anyway. It's probably still a lot better than the 250cc bike and you will not get blown off the highway as easily. Plus when you get older you can unrestrict it and have a full 600cc motorcycle. That 250 ain't going to get it done dood.
250cc motorcycles suck for anything other than off road or city streets. Once you get going about 70 miles per hour on the open highway the wind will kill you, restrict you, bum you out, or blow you off the lane.