WCF, you can install the disk that you want - temperature is not a problem, beside, all 2.5 units are built with strict power consumption (hence temperature) objectives. There are only two real issues:
1st get yourself a 7200 rpm unit - anything less is just too slow for today world.
2nd get an external usb case so you can clone/copy your data without going thru the hassle of installing OS, apps and related disk-joking. Then you will be able to use the 'old' disk in the external case as a backup - always a good idea expecially with laptops.