Hi there! For what it's worth I bought a 2nd hand IBM Thinkpad Laptop about a year ago and it's great: rugged (stands up well to pounding in a Flight Bag), fairly slim and lightweight. Cost around £550 for a machine that was only 5months old.
Useful note: IBM T/pads have a 3 year transferrable Warranty and by typing in the serial number of your particular machine on the IBM www, you can check the machine's background history -- useful for those back-of-the-pub transactions. Also the machine has a fairly quick built in self-test routine which checks just about every conceivable aspect of it- again handy before you hand over the cash.
I bought mine from a guy in Glasgow who specialises in reconditioned/refurbished laptops: Laptops Direct tel 0800-0523548. Ref - Tom Lee. Most of his machines are IBM castoffs from their factory up the road at Greenock. Trustworthy and very helpful. Mention me (not meant to sound arrogant !) & your aviation interest & he'll be pleased to help you out.
regards, Peter Jackson