Being that you are from the UK, you most likely would get an international version ECU (the computer) which does not limit you to the 70 MPH that we are here in the USA. Although, without this limiter in the ECU, you will only be able to go slightly faster at around ~72-73 MPH (stock machine). This varies based upon conditions (temp, altitude, etc.).
Also, in the brand new seadoos, the ECU limits the RPMs (and therefore MPH) until it passes a break in period. Once you are above 6-10 hours on the machine, it will let you see its top speed.
As far as the range, I am unsure for that model. Each ski has its "optimum" speed which gives you the best miles/gallon. I have yet to see complete test data for the Seadoo lineup of skis. In order to get this, you must hook the machines up to a fuel flow meter, which lets you know how much fuel you are running through the lines. You can use this data along with RPMs and/or MPH to determine your optimum cruising speed.
I can tell you at WOT(wide open throttle), you will get slightly under 1 hour of ride time and obviously have a range of just under 70 miles. Now again, your ski has it's highest fuel burn rate at WOT. It is reasonable to say that you could achieve perhaps a 130 mile range at a lower cruising speed of perhaps 40 MPH. I am guessing at this based on similar statistics for other brands/models.