As mentioned above, rust is the most important thing to look out for, Ghiblis rust everywhere but key areas to check: bulkhead area near the bonnet hinges, if its rusty there its best to walk away as its engine/dashboard out to fix it properly. Floors, sills and wheelarches are also particularly prone to rust and also be sure to check under the seals in the door apertures along the roofline. Also check front chassis legs as they can crack. Make sure there are no water leaks in the boot, a lot of Ghiblis have gone sideways into obstacles and will leak if not repaired correctly.
Mechanically these are very strong cars as long as they've been well maintained, watch out for cracked exhaust manifolds which are an engine out job to fix, alternators and starter motors can be weak and you can sometimes experience minor issues with various sensors, O2 sensors being particularly problematic.
As Haydn says headlamps are very expensive and difficult to get hold of, and those that have them want big money. As this one just has a broken lens try
www.maseratispares.nl who have both OEM Maserati replacement lenses in polycarbonate and also some aftermarket lenses in lexan which are a bit cheaper.
The Series 1a is considered by many to be the most pure and raw Ghbili, they are quite a lot lighter than the later cars and with no ABS they are quite interesting to drive hard.