2.5" HDD usually require 500ma during spin up, and sometimes during seek - But during operation about 300ma- This is to conform with USB standards. Although, if inrush currents are supported (or not limited) then it may take up to 800ma on spin up to spin up quickly but will drop down to 500ma within seconds.
So if you putting your Pi inside the 3.5" enclosure, then leeching 5v for the Pi will be fine. since the 5v for 3.5" HDD should allow up to 2A.
The power supply is most probably 12volts (common jack), since 3.5" also requires 12V. The amps on the power supply don't mean anything in relation to the 5v line. It all depends on the regulator inside the 3.5" enclosure.
Unless the power supply is 12v/5v (4 Pin jack)- Then you will have Amps rated for each voltage, with no regulator in the enclosure. That should be 12v/1a 5v/2a
Generally speaking, the Pi will run from the 5v line without any problem when using a 3.5" enclosure, given that you are not using extra things like WiFi, Cameras, etc. LAN will be fine, since its always powered anyway.
If you are planning on putting a USB HUB inside the enclosure, then you are going to run into trouble with power.
If you do find that 5v wont be enough, you may as well use the 12v line and regulate it down to 5v for the Pi. You can use something like an UBEC, its cheap, small and reliable. (and will most probable enable you to use WiFi, Camera's etc with the extra power!)
- 1A/12v gives you 12 Watts
- 12 watts for 5v gives you ~2.4amp @ 100 efficiency. I wouldnt count on more than 2A from 1A/12v using a 2A UBEC anyway.