Bellingham was playing regular first team football for Dortmund at 18, come to thing of it he was playing regular first team football for Birmingham at 16 so there is a slight difference.
This was not a case whereby Spurs decided not to sign him because he was too expensive. No, this was a situation where Nusa and his entourage wanted to sign for a club whereby they were "guatanteed" starts. The only person at Spurs that can guarantee you a start at Spurs is Ange, not Levy nor anyone else.
Nusa made a footballing decision and did what was best for him. At 18 sign for a small premier club where he is guaranteed to play knowing that later on, an elite club could come in for him.
Besides,in 2 or 3 years he may well be a fantastic player (for the record I think he will become one), but there is also a possibility that he might not.