iPod best ipod for playing long files (classical music)?

I'd go for a "big" iPod in that case. The details matter a little more here than they would with most thump-thump pop music, and you may want to use lossless or at least a higher compression bitrate that would eat faster into the limited storage of a flash unit.

The HD iPod models use rather large caches to cut down on disk access, and the cache actually works better to save power if you are listening to fewer long tracks instead of skipping all around.

Weren't there issues with HDs interfering with the sound quality of HD-based iPods? Could be just FUD from anti-iPodders. I prefer flash-based just because an 8GB nano holds more classical music (ripped at 320kbps AAC) than one can listen to in a sitting.

But if you're really into classical music, then a regular HD iPod playing lossless would be ideal. I see this set-up more for an in-home listening session.

I would say flash, because listening to long files requires the HDD to spin for longer to load that long file into the cache. On a flash player it's all just there ready to go so you'd see very little battery-life variance.

That being said I'm unsure as to the difference in sound quality between the regular iPod and the flash-based iPods, I would hope they'd be the same but one can never be sure.

I listen to a lot of Classical Music, I have a 1Gb iPod Nano and it works gloriously for me. You would obviously want to get a larger one if you want all of your collection on the iPod all the time. But I find for my needs 1Gb works great.

I'd go for the biggest iPod as it has the most RAM (64MB). Not only will you have enough space to store everything, you also have a big enough RAM so battery life won't be affected that much, especially if you store in Lossless.
But you probably won't get the full bat life with Lossless AND classical music. I'd say around 70% would be a good estimate.

If you are heavy into classical music I'd worry more about some decent headphones than about what iPod to buy though.