From what you have said about your girlfriend taking and using your K3 (and denying that this is a frequent occurrence) if you do get the K1, make sure
you like it because chances are you will end up using it (as you already mentioned) while she permanently confiscates your K3.
If you can afford it, I agree with others here that you should probably spring for the Kindle 3. Though Amazon is still selling accessories and some replacement parts for the Kindle 1, who knows how much longer they will do so, and they certainly aren't developing new firmware for it.
Still, take a look at your friends Kindle 1 and see if you like it. The article that daffy4u linked for us found that the Kindle 1 actually compares quite favorably to the Kindle 3. If the price is right, perhaps you will find it fits the bill until you can comfortably afford to buy a Kindle 3.
Holly