first of all a Happy New Year to all,
I think for buying a classic the difference with the more recent one is:
it's a seller's market instead of the buyer's market for the modern ones, you have lesser choice and the seller is sometimes the deciding party.
When looking for a classic Ferrari a lot of patience, travelling and gutt feeling is involved.
Buying a classic can be a money pit, there are no cheap ones around in the end. Either you go for a car that's OK in all senses, the money you put in afterwards is less. Or you go for a car which demands some imput (work and capital) from yourself, labour rates at specialised workshops are high, as so are prices for the pieces.
As MOMO pointed out the well known cars as 250TR, 250GTO, LM, and such wouldn't be a problem regarding servicing, maintenance, ... as these were well taken care of by their owners who are seeing their car as a piece of art, but also as an investment. It's more of a problem when you start looking for a more 'mass' produced classic like 330, GTE, even Daytonas.