I've bought a succession of used high end Dell systems. My current laptop is a Dell Precision M6500 with a Core i7-920XM processor, 16 GB RAM, mSATA boot/swap/home, 1TB and 2TB drives in the internal bays, and I've replaced the DVD with a caddy for a second 2TB drive. 1920x1200 screen, Radeon HD5870. It will take up to 32GB RAM.
I like these systems because they're very easy to maintain and upgrade. I'm rough on keyboards, so I've replaced that a few times, and I've replaced the battery. I've had it since 2011, and it was used and a generation behind at the time. Just maintain it; the most important thing is to blow out the fans with a can of compressed air every now and then.
These machines are cheap and plentiful on eBay, going for about $400 with an i7-920XM or i7-940XM processor. It's easy to find parts for these robustly-constructed systems. I've been very happy with it. I run exclusively Linux (openSUSE) on it, and everything works fine. They actually offer Precision laptops with Linux preloaded.
If your old laptop weighed 4.8 kg (10.6 lb), you'll find the Precision series to be light and fluffy. It has a plain silver exterior with an understated Dell logo, except for the "Covet" version which is orange (and functionally is no different).
The only issue is that the processor is slow by contemporary standards. It has a Passmark rating a little under 4000, vs. some of the newer i7 mobile chips that are up around 10000 or even 11000 (I've found that for what I do, which is a lot of photography, software development, and some video, Passmark is an accurate reflection of relative performance to expect, compared against a Core 2 Duo E8400 and i7-5820K).
My other observation from the Passmark numbers is that there was a big performnace jump from the top-end first generation to second generation i7 processors (the top second generation is around 7000), a significant jump to the third generation (around 9000, IIRC) and smaller improvements after that (the top end fourth generation is around 10000; the 5th and 6th generation no higher, and you have to go to a mobile Xeon to get any meaningful improvement, and that's only about 10%). Dell has released newer Precision "mobile workstations"; the 6600 is based on second generation, the 6700 on third, the 6800 on fourth, 7710 on 6th, and 7720 on 7th generation processors.
The big downside (to me) of the newer ones is that they're full HD (1920x1080) rather than WUXGA (1920x1200); that difference is quite significant. This is going to form the basis of a question I will be asking that's closely related to this.
Based on all of this, my recommendation to you would be to purchase a used Dell M6800 on eBay with your choice of processor and screen. It's easy to upgrade memory (to at least 32GB; I'm not sure about 64GB) and storage with just a screwdriver, and memory and 2TB disks are cheap. The processor is more difficult. So if you find a good deal with a top of the line chip (i7-4980HQ, i7-4960HQ, or i7-4940MX) and the screen you want, I'd go for that first and worry about memory and storage later. They offer these systems with lower-end i7 and i5 chips, so check carefully.
If a 17" screen is too big, you can go with the M4800, which is 15" but otherwise similar (I don't know offhand whether it has two drive bays). They also offer a 3200x1800 display for the M4800, but nothing higher than 1920x1080 for the 17"!!!
Good luck!
