I still use it a lot on cars and restorations to blow off rusted fasteners and to heat bolts/nuts for removal. The cutting and rosebud tips in O/A are still the best solution when it comes to delivering heat quickly and precisely. For annealing metal (especially aluminum) in preparation for forming I use a rosebud tip to apply heat very quickly -- although for smaller jobs I now use an air/propane torch hooked to a standard BBQ-Type cylinder.
I haven't done O/A welding in quite some time, but I still do quite a lot of brazing and silver soldering with those O/A welding tips. Just last weekend I brazed a cast iron door flange for a neighbor. It also came in very handy two weeks ago when I heated and removed a corroded O2 sensor from my daughter's Highlander. Of course it still cuts thicker metal much better than my plasma cutter.
There are two basic options -- Professional heavy duty O/A outfits are usually 2-stage regulator units; meaning that cylinder pressure is brought down to working pressure in two separate regulator chambers, and that makes for better flame control and precise welding operation. Cheaper and often lighter duty O/A outfits usually are single-stage; meaning that cylinder pressure is brought down to working pressure within one regulator chamber. It's not nearly as precise as 2-stage, and as cylinder pressure drops, you have to fiddle with the torch settings more often to keep proper flame control.
The biggest issue in buying a used set is to make sure you don't have a diaphragm or needle valve seat leak in the regulators and/or a needle valve problem in the torch handles. Older equipment that's been sitting for a long time can often have diaphragm problems --- ozone cracks and dries the neoprene material. That said, older Harris or Craftsman (usually Harris), Oxweld, Smith, Victor, Uniweld, AIRCO, or Purox (Union Carbide) are damn good systems and will work for decades without any significant maintenance. My Craftsman, AIRCO, and Union Carbide rigs have been working since the 1970s -- no issues.