F355 headers WILL fail and require replacement at some point. Be prepared to eventually shell out for a new header setup if the particular car you're looking at has OEM headers.
If you're really bent on a 355, there are plenty of examples out there and finding a well sorted car shouldn't be too hard. Don't buy a Garage Queen with low miles if you intend to DRIVE her! You can save a few grand by going with a higher mileage "Driver" car, which will run better anyways, and most likely will already have had the expensive major service done!
Have a leakdown test done during PPI, and have your mechanic inspect the headers (if oem) and the valve-guides (if possible) as these are the main problem areas. I'd checkout the 348/355 section of ferrarichat for more info on what to expect and look out for.
I've had a 348tb for several years now and yearly/regular maintenance isn't as bad people make it out to be. That goes double for reliability, as the car has NEVER left me stranded and isn't at all finicky. The major services can get expensive fast, but that mostly depends on the car you buy and how many years you intend to keep it.
I say do it if the car is well sorted and it feels right in your gut!
BMW: 2011 AW/Black E92 M3 DCT - 2010 BSM/Tan E90 335i AT
Other: 2004 AW GDB STi 6spd - 1991 Nero/Nero 348tb 5spd