The interior of a 355 is a simple design, full of Connolly hide and that famous shift gate. A well looked after car will be close to perfect inside, but there are things you should check when looking around the car.
This phenomenon appears to happen mostly in hot climates where cars are left with the sun beating down on them all day. This tends to have an effect on the leather that covers the dashboard, and in particular the airbag cover. There is nothing that can be done to correct this other than to recover the whole thing. This is probably not as bad as it sounds; if the airbag cover only is affected, this can be removed and recovered by itself. One owner in The States has had his whole dash recovered in alcantara so there are plenty of options available.
Here's an interesting thread on the subject: http://www.ferrarichat.com/forum/showthread.php?t=87453
Sticky Centre Console
The centre console parts are all made from black plastic which is covered with a rubberised coating. On many cars this coating has gone all sticky and the only way to correct it is to peel it all off. To date no owner has found a method of refurbishing these with the same finish. Unfortunately it is not cost effective to buy all of the trim pieces from Ferrari to replace them, as one can only be had by buying an air-con ECU at around £700! Ricambi America carry a replacement piece to stick over the top of the Air-Con section. This can be found here.
As always, opinion is divided on the cause of this stickiness, but many people think it is caused by the use of cleaning products which contain silicon. To be on the safe side therefore, it is probably best to stick to just a damp cloth when cleaning the interior.
The original gear knob is a polished metal affair, which has a clear laquer sprayed over the top. The laquer tends to peel off over time and whilst this clearly doesn't affect operation of the vehicle, it is one of those things that you see often and can therefore become a bit annoying. You can either replace with a new Ferrari one, or better still (and cheaper) buy an excellent Hill Engineering replacement (www.hillengineering.co.uk). To replace, simply undo the lock nut underneath and then unscrew the knob from the shaft.
Rattling Gear Stick
Some cars suffer from a rattle coming from around the gear stick area which, if you put your hand on the gear knob, can be stopped. This is caused by a worn bush at the bottom of the gear stick and is nothing to worry about. Correction might be a lengthy process though as access is likely to be from underneath the vehicle.
Leather Seat Bolsters
Due to the shape of the seats, the drivers side seat bolster tends to get more wear due to the rubbing which occurs as the driver gets in or out of the vehicle. Reconnolising of the seat is possible either just on the worn patch or on the whole seat. You can either have a professional do the whole thing or buy a DIY kit. DIY kits are fairly cheap but it is best to send a sample (get a piece of leather from under the seat) to ensure a proper colour match.
The Spider roof is a fairly complex electric affair which has its problems now and then. Most common are the seat potentiometers. These are located under the seats and are used to detect when the seats have moved fully forward. To get the roof down the seats are automatically moved forward to provide clearance, but if the potentiometers fail then the ECU operating the hood can't tell the position of the seats. For safety reasons therefore it won't move the roof. If you suffer roof failure there is a manual wind mechanism that can be used as backup to get you out of a tight spot. Potentiometers are not particularly cheap to replace, but then that is true for most Ferrari parts.
Another common problem with the Spider roof is hydraulic pump failure. This can be very expensive. If a car you are looking to buy does not have a working roof, ensure you determine the exact cause of the fault before taking the plunge, and negotiate an appropriate amount off the asking price if it hasn't already been taken into account.