Livre d'Or

If it's a desktop replacement and you rarely to never carry it around would a desktop be more suitable, keeping the existing laptop as the one-off luggable? More for the money, way bigger screen, much easier to fix/upgrade but obviously much less portable.

For making it overheat less, I’d definitely take it to bits or get/pay someone to do it for you. Whether dust, fan or heat sink paste failure this is all fixable (dust obviously being the easiest if the heat sink is easy to get to. Some laptops are easy to take apart with easy online guides or maybe even the official service manual. Some are... much more of a pain in the behind).

I'm a fan of mid business and high end range Dells (the things are often easy to repair and the next day service package is fantastic) and high end Lenovos (expensive but nice), but these both cost more than you want to spend. Apple makes great hardware and you can dual boot it with Windows so everything can be made to work natively and you could ignore OS X completely if you wanted, but again more than your budget. Otherwise I think it's the luck of the draw. Every manufacturer has made great laptops and dud laptops and often you just don't know how good it will be in the long term as by that point it's been replaced.

OS wise, all three major OSes work. I use Windows as my host OS, but do most of my work in a Linux VM, leaving Windows for Internet/email, Office, photo editing and gaming. Windows 8 isn't as terrible as many make out (see my post above), but everything is just a case of getting used to it. The ease with which you can run a different OS in a VM means OS choice is practically less important. You could use Windows as your main OS because the hardware works and run everything in a Linux VM if you wanted, or vice-versa. Same applies to Macs as well as PCs. That said, it sounds like you probably just want Windows 7 as it's probably closest to your XP experience?