Get her a Lenovo business laptop. Yes, they are a little bit pricey but you get what you pay for.

A lot of people on here are trying to give you parental advice.

That being said, here is my take on the situation and your request. Like many others I think that what you want is over the top for many reasons. The amount of time your going to spend monitoring and up keeping the network is going to be become more time than your going to want to spend, very quickly. It would be far easier for you to lay down the law with your kids and explain what they can and can not do.

I would then go with the basics.

1. A scheduled virus scan on each computer for when they are at school.
2. Router with DD-WRT so you can set Internet access permissions when you are away.
3. OpenDNS so you can restrict many sites including proxies, adult content, etcetera.
4. Rewrite dns queries to opendns.

You have to remember that your kids are going to find ways around any setup. It just depends on how much time you are willing to spend setting it up and keeping it up.

I will say that, and I'm sure many will attest to this, having your own computer at the age of 15 opens up a Pandora's Box. You are taking a huge step in giving them their own computers, let them know this.