The trouble is going to be that laptop specs (particularly Windows based ones) change so quickly that the advice today isn't necessarily valid tomorrow. A minor change (say a different video card with different drivers) can have a knock on effect that causes dropouts in your DAW.
However, some general advice:
First, you don't need a Mac. Fans say Macs are more stable but, if that's true, it's only because Apple control the hardware and software being used. You can make your Windows machine just as stable by not loading it up with every bit of free junkware out there. You'll certainly get more bang for your buck with Windows.
Second, get the fastest/best processor you can afford. Frankly, the hardware has now caught up with DAW software and even my 4 year old laptop can do fine mixing up to 30+ tracks with effects--but it's still worth getting the best you can afford if only for future proofing.
Third, get as much RAM as you can afford. RAM makes a big difference to how much you can process at once. 2 gig is a bare minimum; 4+ gig is better; 8+ better still.
Fourth, ask about the disk speed. A lot of laptop disks spin at 4800rpm. This limits throughput with things like audio tracks. 5400 should be your minimum, 7200rpm is better. Faster again is even better but very unlikly you'll find it as standard. However, you may find that you can circumvent all this by using an external hard drive.
Fifth--and very important--check the laptop has the right connections for your planned interface. Pretty well all have USB these days. Make sure it's USB2. Maybe you can get USB3 but not likely for your budget. If you have any idea of using a Firewire interface or external drive, you're going to have a very limited choice of laptops.
Sixth, and a favourite of mine, look for a laptop that supports a second monitor display. DAWs like screen space and as soon as you get into serious mixing, you'll want to add an extra big screen monitor.
That's what I do. I drink and I know things.
-Tyrion Lannister (and Bobbsy)