Is using Time Capsule better than using an external hard drive because it won't stress the hard drives? (for using Time Machine)
That last paragraph is very difficult to understand, but if I follow, creating a RAM disk is not going to help...primarily because there's no problem to fix in the first place; Your concerns are unwarranted. 

Time Capsule hard drives in particular are very low failure rate, and above that, Time Machine does not tax your system indefinitely. When the timer starts a new backup, the hard drive will spin up. On completion, it will spin down as necessary. These drives, like most of decent quality today have a high MTBF &amp; MTTF, so relax.