R12 is readily available on craigslist everywhere. Just buy it with intent to resell and sell it to your mechanic for $0.01.
No w124 came with parallel flow. They are both tube and fin. We have the worst and that's why our conversions just plain suck. Its commonly thought that the later condensers were bigger or better, they aren't; they just changed the location of the hoses and added the more powerful 23/25 amp twin fan assembly with shroud.
Here is the later model condenser -- tube and fin:
