Just as a note if you go the item of power route: Don't make it grant the "breath weapon" power - it doesn't need that; it gets its usability as a ranged weapon just off of the item of power "It Is What It Is" entry.

+3, as noted by others, is probably a good baseline, assuming +2 availability & +1 researchability - however, were I GMing, I'd consider going up to +4.  Here's why.

RAW, you get a +2 from having your toughness powers only protect against X, and a +0 for the default of protecting against everything except X.
When X is, say, fire, this is easy to adjudicate: +2 for your powers only protect against fire, vs, +0 for your powers protect against everything except fire.

But consider: which sounds like the better description here:
Your toughness powers only protect against non-living attacks.
Your toughness powers protect against everything except living attacks.

They both sound pretty reasonable, really.  So I'd be tempted to award an extra +1 for, in essence, having a really broad catch.  It's not quite broad enough to get the +2 for only protecting against a limited range of things... but it's far broader than a normal +0 type catch, which could still be something very narrow like fire or cold or iron.

Well, the real problem here is actually that the RAW is exceptionally unclear.  As noted, there's exactly one side-comment in one side-bar that (arguably) supports one position on this.  Thus, I feel this is - either way - at least pseudo-houserule, since someone could honestly think the RAW went in either direction.

Hmm, not sure we're reading it the same way.  Harry states the spell would be more difficult for him to cast because of the block.  Doesn't this directly back up blocks as reducing actions?

The exact words there are that, because there's a five shift block in place, he needs a +5 total to control the spell that'd normally need +3.
By my interpretation / houserule, it would instead require +8 - five to overcome the block, and another three to actually control the spell.  ...Now, I suppose you could read that as, he needs a 5 to get any control at all, and could still eat backlash or fallout for the last three.  In which case we're back to there being no evidence for any interpretation over another.

Looks like the big difference is ease / speed of repairs - though that could probably stand to be spelled out a little clearer, perhaps with specified time intervals for varying levels of consequence?

Actually, I'd say Living Dead covers the "they just keep coming" factor.  You put a few bullets in one, it drops... and as soon as that scene ends, it's up and shambling after you again.

I guess I don't equate "likelyhood to hit" with precision.  A perfectly aimed laser beam might be just as hard to dodge as a huge fireball, just for different reasons. Or they might both be EASY to dodge. It depends on the circumstances, who is attack, who is defending, etc.

Hm.  This is actually a pretty good argument in favor of just going with high conviction & discipline for such a character, and using an aspect for raw power vs. finesse.  Thanks.