Problems as a daily driver? I can only think of normal running costs and petrol. Parts aren't too bad in price (as long as you are used to the prices of parts for high performance cars) but remember that you need twelve plugs, 18 litres of oil, two oil filters etc. plus labour can be expensive simply because some jobs take a long time. Petrol will be around 10-12mpg unless you boot it around and then you can expect single figures - this is for a 400 though, a 412 ought to be better.
Main points to watch are the exhaust ($4,500 in a specialist in stainless steel, $7,000+ in steel from Ferrari), tyres can be hard to find if they the car is still metric wheels (supply has become easier recently though), rust on the body, door bottoms, the boot floor/rear wheel arch area and the front chassis outriggers, some parts are almost impossible to find and there are very few 'spares' cars around and those that are usually have the same problems as the one you are trying to repair anyway, trim work can be expensive (and hard to find good trimmers), The engine, auto gearbox, back axles are pretty bomb proof. Brakes, wheel bearings etc are pretty much as you would expect. The 5-speed is rare car and as far as I am aware has no real gearbox problems but a slipping clutch could prove expensive to replace (I'd guess at $2,800-$3,500). The auto boxes rarely give problems.
The above is a very brief rundown off the top of my head. Is this your first Ferrari? I would suggest that you get a specialist to look over anything you buy, this can be a good investment. Service history is more important than milage, Ferraris that have sat around for a long time develop faults and can be expensive to put right, cars that are used are usually better buys.
Have a look at www.400register.com which has some useful info on it. Especially look at #21557 in the 'Register' section. This is my 400 which I restored and was just an normal 400. It will give you more of an idea about the cars.
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