Howard,
A warm welcome to the forum and let's hope you get a good car sorted out for yourself soon. I have had a 2007 4200 Facelift and a 2007 GranSport, both are excellent in their own way. The 4200 facelift is very close to a GS in many ways, but not everything. They have a different interior and slight differences to the engine configuration. Look for the normal hole, or rust under the offside rear light, a common issue, check under mats in the car and boot, make sure it is dry, they can leak. Whatever you decide to make an offer on, insist that you have an Independent Inspection. There is a list of Indies on here that cover the country and for £200-300 it's well worth the money..!! Try and start the cars from cold if possible, listen for possible fast engine rattle for a few seconds, it's not serious, normally it's an oil pipe that needs replacing.
It depends on what you can afford, get the best you can for the money..!! In my opinion the GS is slightly better than the 4200, but is more expensive, saying that the 4200 is a great car and get the CC box if you can. Check what life is left in the clutch, a new one is £1500. Check service history, make sure all services have been done by either a dealer, or a Maserati Specialist...important..!!
You could give Richard Grace Cars a call, he has excellent stock usually and is based in Chester, if he does not have it he can find it..!!
The very best of luck in your search..!!
Cheers
R