An imac has neither a PCIe nor Express interface, so it is not possible to use a UAD card with one. You either need a Mac Pro for the PCIe version, or a Mac Book Pro with Express 3/4 slot for the UAD-2 SOLO/Laptop card.
there are many excellent native AUs out there which are easily on par with the UAD plugins and will run on your iMac. I suggest that you take a closer look at waves, sonalksis, psp - to name just a few.
Hmm easily on par with UAD.
I suppose everything is relative because I have both the Sonalksis and PSP stuff and I wouldn't say they are on par. They are something else (and very different from each other too) but for certain things I find it very hard to find anything that is on par with UAD1/2.
Of course if you have to do without them it's perfectly possible, no doubt about that, but that's not the same