Check out used laptops like the Dell outlet. anything they sell today would work.
If you need a serial port (moates doesn't need it, but many wide band sensors like stuff from Innovate do, though that's changing) you ideally want a machine with a Express Card slot or a PC Card slot, since adapters for them that provide serial ports are more reliable than a usb serial port.
If it is a laptop built in the past 5 years, it will have enough CPU power to data log just fine. For reliable performance you will want some way to power the laptop when in the car, either an inverter, or a 12v power supply for it. During data logging, battery life will be reduced significantly. (screen is running, drive is running, usb ports are going crazy, and if you have a serial port it's going as well).
A bigger screen can be handy to make it easier to see things when you are working with it. But you can get by with a smaller one.
If you are doing this just for your car, I'd consider something like a RAM mount to hold the laptop and support the screen, especially if street tuning. Looking at the laptop in the passenger seat is a pain if you spend any time doing it, and picking it up and hold it in front of you usually tangles a bunch of wires you have hooked up. Mount it with a system in one place that makes it easy to see FTW.