Samuel23, all of the points you named can be a trigger for committing these type of crimes. I think the guy they are looking for will be about 35 years old and single.
I think he could certainly be responsible for the previous crimes of the late 1990's and early 2000's and has recently been released from prison. I also think that this will not be the end of it, he is on a roll and will want to continue, most crimes committed by serial killers tend to gather in intensity, so I feel that he will move onto another area and continue. It wont be too far from where he is operating from now - maybe somewhere like Peterborough, Cambridge etc
The other option is that these crimes are being committed by a lorry driver operating from Felixstowe Port, and then of course we could be dealing with a foreigner, but I think it is more likely to be local, as they have good knowledge of the area. The only thing that would put me off this idea is that the murderer would have to have a second vehicle, as a large lorry would be more than noticable.
I know there is going to be a police conference at 11am, when they are going to formally identify the 2 new bodies, I am hoping, in a sad way, that it is the 2 missing girls rather than 2 new ones, otherwise the total will be climbing to 7.