Macs don't overheat. This is a fallacy that nontechnical people perpetuate because the metal casing gets warm (that is, in fact, the computer dispersing heat as it was designed to do).
You didn't say if you mean 1TB of hard drive or 1TB of SSD, but the latter would certainly not impact battery life in the slightest and the former would have more of an impact but not significantly.
Loading up and maxing out a new computer can HELP future-proof it but is no guarantee that it will last longer than one that is not maxed out. Bigger hard drives do not magically fail less often than smaller hard drives, for example. Certainly, if you can afford to max it out from the get-go, that's great.