They key issue you have is
"My laptop gets a signal at the house, but nothing else does"
To work a repeater must be able to get signal in the first place to be able to repeat it. Then if it is not receiving a good signal it will just repeat that plus any other new errors you get between the repeater and the devices in your house.
What you want to try to buy is everything N. If you have anything that runs G it will slow everything else down to G. The 300m does not really matter since as you point out you only have 3m. What is important is that everything runs the same, you can run a mix but the end result will be the lowest common denominator.
The USB adapters for PC are simple to get installed. The extender is another story. You now have 2 radio networks you need to deal with. So you now have issues with SSID and encryption keys times 2.
I would only use a extender as a last resort. If you have any option to use a ethernet cable it is your best bet.
At the distance you are talking it is likely you will need to use special antenna on the extender and/or you linksys. Mostly this is just learning how antenna and AP and Extenders and routers all hook together.
My business is based in my workshop, about 100 feet from my house. I have a Linksys e1000 router (300 mbps) in the workshop hooked into DSL (3 mbps), but I'd like to link to the house as well. My laptop gets a signal at the house, but nothing else does, so I know I'll have to get a wifi extender. Also, my desktop computer in the house will need a wifi usb adapter. So, my question is this: does the repeater also need to have a 300mbps capacity for compatibility sake, or can I go with something slower. My max internet speed is only 3mbps. Also, re: wifi adapters, are they as simple to use as they seem? Just buy one, plug it in, and done? Or are there compatibility issues there as well?
For the record, we'll be connecting the laptop, the desktop, 1 DVR, 1 game console, and 1 handheld game -- certainly not all at the same time.