Answering some of your questions:
Buying through Scubastore may mean that you have to send it back to them in Spain for warranty repair. Not a huge issue as long as you're willing to pay to have it serviced locally. The other thing about the Cressi reg you linked to is that it's a DIN 1st stage - most U.S./Caribbean non-technical diving is done using the more available Yoke configuration. (most regs you see on U.S. mfr's websites are yoke) You can also generally convert either way if needed with an inexpensive adapter.
What I'd really suggest you do - unless you plan to learn how to service your own regs - is find out what brands your local dealer can service. And buy one of them. I pay $60 every other year to have my reg serviced.
Don't be too taken in by the plastic vs. metal debate either. The top of the line Atomic Aquatics T2x ($1600) has a "plastic" (ABS) 2nd stage. As does the $1500 ScubaPro MK25/S600T. It's done to minimize weight and thus jaw fatigue. I've had people set tanks on my 2nd (accidentally) with no consequences.
The Aqualung Titan LX 2nd stage is ABS. Aqualung is a solid, reliable brand that has been around for decades.
An easy way to tell plastic vs. metal is that if the 2nd stage body (not the face) looks black - it's ABS. If it's shiny it's metal. Compare the Scubapro S600 (ABS) vs. A700 (Chrome over Brass?) to see the difference. Many are some combination of both.
For recreational use, you'll probably never exceed 40% Nitrox. Most boats can't pump more than 32% and low-30%'s are about all you can safely dive at recreational depths. I'm stressing
recreational so I don't start any debates.
You won't be able to get free service on any reg you buy from eBay. Unless you buy it
new from one of the few authorized dealers who also have a storefront there.
Most manufacturers (including Aqualung) will not warranty products purchased there. I believe Aqualung won't warranty products purchased anywhere except through it's authorized dealer network.
For discount pricing with full manufacturers warranties check
ScubaToys - Scuba Diving Equipment Discount dive gear Snorkeling Equipment and Wetsuits Cheap online. Or
Buy scuba gear, scuba diving and everything underwater from LeisurePro.com - but only some of their lines (Oceanic for ex.) carry manufacturers warranties, others will only be warranteed by LeisurePro. It's all spelled out in the fine print on their website.
It's not a huge issue unless there's a fundamental product recall/replacement problem.
Mouthpieces are replaceable. They start around $7 and the comfort bite ones go to around $25? A lot of them are interchangeable - certain brands use a slightly different size.