Is this a good idea any more?

So, I've been meaning to get a laptop recently for various reasons, and don't really want to spend much on one, so I've been looking at the old business class laptops that float around on ebay that are a couple years old (if it's not blatantly obvious I'm not really looking for anything with high end graphics or a super-high-res-9001k screen) - and two in particular caught my eye. HP's elitebook 2570p and the dell latitude E6430. Why? Because they cover one of the things I hate about laptops nowadays - they're not black boxes of soldered down, booby trapped magic that can't be modified to any further extent than maybe stickerbombing the chassis or something ridiculous. These ones in particular, don't have soldered down CPUs - they can be replaced. They both seem to have a history of people throwing quad core i7's in them (see: link and link), albeit at reduced clock speeds for the 2570p.

I like the sound of doing this - the CPUs they accept seem to pop up from time to time on ebay also. I just wanted to check if getting one of these and doing this is actually a good idea any more? And if it is, also, as to whether or not the bigger form factor and the (IMO) uglier design of that dell is worth the extra MHz, I'm undecided. Halp?

I got an E5530 with an expanded battery some time ago and i love it. 8 hour battery life and a decent amount of horsepower.

Working with laptops, especially smaller laptops is a royal pain in the ass. Be well-prepared before going into this, and I'd suggest going with the bulkier Dell.

I'm not too worried about disassembly to fit new components, there's manuals available that cover everything for both laptops, here and here. Apparently the dell's fan is uncontrollable.

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I would go with the Dell. While the Dell's fan isn't manually controllable, the HP's fan will be on full blast all the time anyway because HPs have a nasty habit of overheating. Dell at the very least has always known how to make solid, business-grade laptops.

I have an instinctive, habitual and involuntary aversion to HP after the debaucle with my mother's dv6000 SE. I've seen so many HP laptops die horrible, strange deaths thanks to their poor manufacturing, just the weirdest ways that those laptops have given up the ghost... Dells, on the other hand, I've seen dozens shrug off years of abuse. My dad's old work laptop was a Dell Latitude D620. That thing ran nonstop for years and years and years and not once yet has it given any indication of age other than that its battery is no longer considered a battery.

the cpu's are replaceable but you'll have trouble finding them cheap. the ones you spoke of will cost you probably more then the laptop themself. also upgrading the ram is not really optional and you should probably get a SSD, so after the cpu, ram, ssd, and maybe a new battery you'll still end up paying around 600-700. you'd think the prices on things like the ram and old cpu's would have gone down but they are they same as when they first came out.

I have a spare SSD and some RAM already, so that's not a problem. The CPUs I was looking that are floating about are around £100 (usually a bit less), £125 tops I think I saw. They seem pretty scarce but I'd get one some day.