If you only have a USB adsl modem at the moment then you will need some new hardware. The best way is to buy an adsl modem wireless router. Some now come bundled with a USB wireless adapter or a cardbus card adapter. Make sure that your laptop can take cardbus cards.
MIMO is a yet unadopted standard and is more expensive than wireless-g. For a flat, I don't believe that it is necessary as the distances you will be working are not great.
You can use a printer over a wireless network. Set it up as you would for file-sharing over a network. Provided the printer works from the pc it is connected to, it will work over the network. Clearly, it is difficult to work the scanner part of an all-in-one over a network :)
You can buy printers with ethernet interfaces which attach directly to your router. Their only advantage is that you wouldn't have to have your main pc running to use the printer from the laptop. They are usually more expensive and I don't know whether sensibly priced all-in-ones come with this yet.