buy a new battery, add some ram and an ssd to the plastic MB and it will be as fast as the new stuff. alot cheaper imo.
$130 apple battery (or oem if you want to save some $$)
$64-96 for ssd - she doesn't need a huge amount of storage thats what the cloud is for.
$50 for 8gb of ram, half that for 4gb
good to go for another 5 years.
I buy them in that condition for like $200 - spend $100 on a new battery,drive, ram and sell them for $450-500 to friends.
The macbook air 11.6" core2duo should go on MAD sale in a few days since the i5/i7 will replace it - that may be a good solution... either way way a few days for the new air's to come out. or just fix up your mac. its just a pc. nothing special about it.