Hey James,
Some of the newer UAD plugins do take a lot of DSP power, so for example you won't be able to run more than one instance at a time of the Manley Massive Passive on a single Solo card. However, a lot of the oldies-but-still-goodies aren't so resource-hungry. You can get an idea of how many simultaneous instances of a plugin you can run here:
http://www.uaudio.com/support/uad/compatibility/instance-chart.html As you can see, a Solo card will run up to 24 stereo instances of the "Legacy" (original) LA2A or Neve channel strip, which are still great plugins, but only 12 of one of the new LA2A models or 3 API channels. You can always render tracks to free up resources, of course.
If all you want to do is run the older plugins, a Solo card should be sufficient for most situations. For more intensive applications, though, one of the more powerful cards might suit you better. Note that every card comes with a pack of "classic" plugins, including the Legacy 1176, LA2A, and Pultec, plus a usable reverb and channel strip.
Let us know if you have further questions ... have fun!
Cheers,
Eddie