The app K-9 mail is a 3.2 MB download. If you have a 32 GB Turbo, that is less than .01% of the storage space. I guess it will use some data as well, but no more or less than the stock mail app would to store your mail (and presumably you will just delete your account from the stock mail app.)
I think I'd give up so trivial an amount of storage space for an app that does what I want rather than struggling to use one that is inefficient. And if it turns out that the Lollipop mail app adds "select all" (I kind of doubt it will; the stock mail app tends to be getting worse, not better), then you can just delete the third party app and start using the stock mail app again.
You'll lost 3.2 MB - the size of one song, a little less than one photo. Isn't that worth it?