For $100, just get a new Modem\Router combo, much easier to set up.
So: Wall Outlet > DSL2+ Modem/Router > Wired to desktop, Wireless to Laptop, Wireless to mobile devices
Make sure you get a wireless N router at 300mbps, better range (~50m) and goes through walls
Setup is usually pretty easy, unless you want to 'tweak' it.
As you are not 'tech minded', focus on easy setup brands like Belkin or D-Link, I have a Billion which are good but not easy to set up.
One the back of the modem, get a 4 port router (for home they are either 1 or 4).
Your desktop will be on 1, but you may need the second occasionally. I have a printer in port 2, so I can print wirelessly from any device on the network.
For example, I have a 4 port modem router, connected to it wirelessly are 2 laptops, 2 mobiles, a tablet and a TV. Wired are a desktop and a colour laser MFC printer. All running at the same time, no issues. Multiple devices browsing won't cause network lag, unless one is downloading torrents or other traffic hungry data.
Also, tell the daughter to do all app downloads and updates at home to save draining the mobile data limit