1D == very old ???
wow... i feel ancient...
As already mentioned, 1D is a pro camera in terms of build quality, features, and an excellent metering system. The disadvantages are the typical obsolescence that occurs rapidly with technology.. as such the 1D doesn't bring much to the table in terms of sensor megapixel count and digi-processor (noise included).
Then again... a lot of those disadvantages area moot point since your final output is web, 4x6 prints, and don't expect to shoot above ISO 800. I can see a few concerns:
1) Pro camera means that it was used professionally. Meaning photographers used it for WORK which means no babying the camera and quite possibly brought in less than ideal working conditions. My 1d-MarkII was a studio camera which gave me confidence in its shape as opposed to... letsay.. a war journalist's former camera.
2) The 1d-markII reported the number of total shutter actuations embedded within the EXIF data of a frame. I'm not sure if the 1d MarkI did the same but you should check the web for information regarding shutter actuations. If the camera has a lot of actuations on the meter, you might want to consider passing. Unless you are willing to repair take on the cost of a replacement shutter (which isn't too bad with Canon bodies).
3) Batteries are awesome in the 1 series cameras... very long life and very long running time. On the other hand, they are expensive. Consider that as part of your purchase. (don't forget to maintain them... recycle charge them occasionally) These batteries are no li-ion they are Ni-MH batteries.
4) Finally... it will spoil you. Once I went to the 1 series film camera, I wanted a 1 series digital camera. hehehe lol. ... G.A.S. is more likely to infect you in the near future.
Which one would I recommend to you? Hmmmm... Unless you are a journalist for a newspaper or web media outlet (build/features/metering) and/or shoot sports (fast AF, fast fps, metering), I'd more likely recommend getting a newer camera such as the 30D. The technological differences between the two in terms of sensor and digiprocessor is going to be more beneficial than getting into a 1 series body. 1D is a very capable camera if leveraged/used correctly in a limited way due to its age.