Was Vader already going to be in a suit before he was injured? Many dark side warrior such as Darth Revan, Starkiller, Darth Bane, and other Sith that specialize in close quarter combat wear armor as well. Reading the wookieepedia page on Vader's armor, I notice that his armor most likely used Sith alchemy along with many custom features which probably would take a long time to make. Could it be possible that Vader already planned to use an armored suit as a Sith, but as he didn't plan getting his limbs cut off, Palpatine was forced to rush its completion? Although the suit had some advanced features such as the mandalorian crush gauntlets and as well as audio enhancers and sonic dampeners, a lot of other parts were pretty low tech. Was it possible that the suit was in its prototype stage as purely armor when Vader lost his limbs, then quickly converted with life support systems into its final form.
The suggestion is that much of it is custom-made for Vader, albeit we do know that Palpatine and Dooku have had access to some pretty impressive droid design where Grievous and the Geonosans are concerned;
The film's official novelisation indicates that the transformation took rather longer than the editing in the movie would suggest;
The newly renamed Emperor Palpatine Surgical Reconstruction Center on Coruscant, a hypersophisticated prototype Ubrikkian DD-13 surgical droid moved away from the project that it and an enhanced FX-6 medical droid had spent many days rebuilding.
It beckoned to a dark-robed shadow that stood at the edge of the pool of high-intensity light. “My lord, the construction is finished. He lives.”