Welcome to the site! I will tell you what I've learned about Ferraris. First you need to drive them! The worst car you can find is a LOW mileage Ferrari. I also am a member of the Ferrari List (Ferrarilist.com) which is really just an email tree. What I’ve found here are some very proud owners who will defend their models to the dire end. You’ll get a little less cheerleading on the list.
The second thing I’ve found is of all the Ferraris out there the 355 is pretty close to Bullet Proof. There is a consistent problem of the “overheating” cat on the right bank of the exhaust that shuts down that side of the engine. This, to me, seems to be a problem with the electric connectors. Other than that, it’s a car. Drive the thing! You’ll wind up with more maintenance if you let it sit than if you drive it. Even Enzo Ferrari himself stated this. I personally think 12k a year isn’t too much. Your maintenance intervals will come a little sooner but look at it this way. If you’re driving the car say 10,000 kilometers a year you’ll be replacing the belts every 30k or in 3 years. If you’re only going to drive the car 3,000 miles a year you’ll be replacing the belts in 5 years. “But, 5 years only calculates to 15,000 miles.” You say. True, but, rubber that’s not being used (flexed and all) gets old fast and 5 years seems to be the time most people say they should be replaced.
Look for a car that’s been driven and has only been fed Synthetic oil and replaced every 3,000 miles. Mobil 1 is my recommendation. Synthetic oil reduces wear on parts far better than standard oil. Reduced wear means your engine is going to last a lot longer. If the car has had proper maintenance you shouldn’t have a problem. I don't see any reason these cars shouldn't have to have a rebuild untill 150,000 to 200,000 miles.
To me the perfect number to look for on a car your going to buy is 3,000 miles a year. It’s enough mileage to keep all of the rubber and bearings in good working condition and low enough to keep a good resale value should you decide to sell it a couple of years down the line.
Having said that, if I had to choose between a 1998 with 18,000 miles and intermittent maintenance records and one that had 40,000 with full documentation, I’d go with the 40,000 fully documented car. Someone who’s anal with their paper work is going to tend be as anal with the way they drive and tend for the car.