Answers .....
The EL1500 and the EL2150 are nearly the same, except the EL1500 is a mono amp, it only has one signal channel, and the EL2150 is a two channel amp, stereo, that has the option to be bridged for mono use. From that I would suggest the EL2150 since you could use it either way. Always figure out your whole system and how you plan to wire it before you buy the amps etc. to make sure it can handle the loads etc.
How many speakers you can hook up, that depends on the impedance of the drivers and how they are wired. For example two 4 ohm speakers might be wired in parallel to present the amp with a 2 ohm load.
Almost NOTHING is really refurbished, products get returned, checked out, and resold. Damaged goods go to the crazy mo part of the company.
There is NO RIGHT SIZE BOX. Box size and speaker parameters determine the tuning of the box, the frequency which will have the most output and the Q or shape of the response curve. To make the speaker sound best in your car, you need a box sized so that the tuning is complimentary to the tuning of your car interior. OTOH just buy a dam box and see how it sounds, what I did.
I'm sure bad wire must be out there to trap the unwary, but Home Depot most likely has very good quality wire for pennies compared to a car stereo store.
Phoo, got my first batch today and the 1204 has the rim bent enough its easy to see. Thank you UPS I guess. I may just give it whang and see if it will flatten out. Anybody get the manual, I am pretty sure I hunted down the list until I found one that "said" it included the manual, but nothing in the box but the speaker.