Most important here is:
Brand - Buy something good. I love IBMs, but others like F-Siemens, Sony and Toshiba also do good stuff.
CPU - Get a box with a cpu that requires less energy. AMDs usually get alot more hot than a equivilant Intel cpu.
No laptops are harmed by being turned on 24/7 - I have many and I never shut them down. Heat often isnt any problem either, but working in direct sunlight isnt recommended...
