Not that I'm an audio expert, but....
I agree with NOT cutting original wiring - especially multi-wire connectors as behind radios etc.
That's not so much for any resale value, but to save later hassles for yourself or the next owner (ie, you if the last owner cut stuff).
Instead use adaptor cables etc.
I presume you mean to NOT use the existing sub. IE - why use 2 subs etc. But you want to use your existing speakers (ie, the L & R, not sub).
That could be tricky - matching new amp power to existing speakers etc.
You may be better off replacing the lot.
Of course, the above depends on WHY? Does it have insufficient volume? Poor sound? Or do you just want to $spend?
Incidentally, removing the radio should not mess up the dash. There will be a way of doing it without damage or later rattles.
And I'd probably start with the HU. If that is not good, then nothing else will be.
Besides, does it have the modern iPod and front USB & stick features? It would be a pity to have to re-change your downstream stuff if you change the HU later...
And I congratulate you on looking before you leap. But this site does attract quality people (myself excluded of course!).
PS - An LOC may not be required. It depends on your final setup. Amps might use HU line outs or speaker outs.