The models you are considering are very different cars. I was in the same boat a year ago - 3200, 4200 or Gransport.
I quickly discounted the 3200 on hearing too many reliability issues and non-availability of some parts. Its a great car, but in many ways was superceded under Ferrari ownership who threw out the 3200 engine and jointly developed the 4200 V8 which on all counts I am hearing is a far more reliable unit given Ferrari sunk shed loads of money to improve total build quality and reliability.
The 4200 has timing chains, so no cam belts to change, leading to (hopefully) lower lifetime maintenance costs.
Either the 4200 or GS will be fine if you get a good one to begin with. Mileage and history combined with condition should be your initial guides. A private sale if considered really warrants an independent PPI.
There are many satisfied owners of 3200 and they will vouch for what an excellent car it is. Its my personal choice and view and what has been confirmed to me during my one year ownership talking to a very good reputable Indy that I ruled it out first in the selection process. Of course the noise, the kick of twin Turbos and those boomerang rear lights ignites the visual and auditory senses. But it was not for me as per what I had on my shopping list. In fact I'd probably echo a remark i have heard from a few people - that the GS is probably the best car Maserati made before they moved to volume production in the Gran Turismo. They are scarce in number, seem to be high in demand and therefore represent depreciation-free motoring as prices climb steadily.
The facelift 4200 is the closest to match the GS and avoids the c£6k-£8k premium between it and a GS LE.
Good luck with whatever you choose. Don't discount the 3200 but talk to a few reputable indys about all three models before you decide. That's what I did in my process to Maserati ownership. Hope this helps.