Contrary to what you may think, the regulator isn't designed to maintain constant voltage under all conditions. Alternators are very sensitive to heat, so the prime question is how long the engine had been running, and what the ambient temperature is. On a cold winter day and a light load, your alternator may put out 14.50 volts or more, but that will drop to 13.5 on a hot day. As the alternator heats up under load, the outputvoltage will drop. The attached graphs are from a Bosch technical document.
The current output of the alternator will also vary with RPM, and is lowest at idle. Thing is, low speed operation may not improve much with a "bigger" alternator". If the demand is for more amps than the alternator can deliver, you will have very low voltage. I'd actually suggest you try a couple of left brain things: disassemble your fans and clean & lube the bearings for one. Maybe install new headlight sockets as well. It's also pretty easy to clean the alternator slip rings by removing the regulator and going over them with abrasive. In the end, it will be what it will be.
In answer to your question about reliability, I recently had three rebuilts in a row fail. All seemed to fail for the same reason...the rotor wiring was torn apart by centripetal force. If you take one apart, look at the wiring for the slip rings...out of the factory, the two wires are epoxied to the iron lobes of the alternator. On my rebuilt alternators, the slip rings had been replaced and the wiring left loose. I finally resolved the issue by picking up a good used rotor from a local rebuilder. If you buy a rebuilt, I strongly urge you to open it and epoxy the slip ring wiring.
http://www.peachparts.com/shopforum/diesel-discussion/380379-alternator-mystery.html