Rob,
I will have lots more to follow, and I'm sure that others will add to it as well.
The 'J' is the term used to describe the lip into which the tire bead is seated. Although I am no expert, I believe that all modern rims are of this type, which is required for use with tubeless tires.
Any rim you buy today in the aftermarket will be of this type, so don't worry about it too much.
The concerns you should have are with respect to back-spacing (offset), ensuring that the wheel/ tire size is of similar diameter to waht is currently on the car (otherwise, your speedometer and odometer will read wrong, and your final gearing - speed in gear - will change), ensuring a good fit under the wheel wells (no rubbing on bumps or in corners) and how the change in weight of the new combination affects your handling.
In general terms, your Spiders wheels can be changed for 15 X 7 wheels very easily and most companies selling wheels specifically for Alfa Spiders have already investigated the offset issues. Some popular choices for your car include Panasports, Serpent Motorsports, and many other wheels available from popular Alfa distributors like International Auto Parts (
www.international-auto.com), Vick's (
www.vickauto.com) and Re-Originals (
www.reoriginals.com).
Good luck and stay tuned for more info as I and others add to the thread.
Cheers,