[1] Easy: Is there some special feature of a lens that makes a "condenser" lens? Does being "Plano-Convex" make the lens a condenser?

[2] Hard: What optical characteristics are we looking for in a pre-condenser? Such as: diameter, thickness, focal length, etc.

[3] Easy: What difference does the "edge thickness" make? I've seen it in the pre-condenser calculator, but don't see why it matters...

[4] Hard: Is there any way of finding the focal length of the lenses inside this from Surplus Shed: PRECISION 100MM DIA 2 ELEMENT MTD AIR SPACED CONDENSER 100MM FL? Here is some kind of spec sheet if it helps, look for item number: 01 CMP 025.

[edit]Is there any blaring reason I should not buy the lens above?[/edit]

OK, I've cut it down to one main question: Should I buy that thing or not?

Welp, I ordered it... I'll see if I just threw $31 in the toilet after I receive it I guess...