To add to some of the comments above, other reasons for a dual extruder are for multi-color prints, and also for using the second extruder to print dissolvable support structures when printing an object with overhangs.
As for build volume, clearly that is based on the objects you want to make. By your initial description, build volume would not be an issue. In my personal usage of my 3D printer, I have often wished I could print larger and have rarely wished to have dual extruder. My next printer will definitely have a larger build volume. And if you use a quality slicing program for your prints, easily removeable support structures can be created thus reducing the need for a second extruder for support material.
As for printing nylon, which I have done, you need to clarify what "short term" means when talking temperature. I print nylon at about 250-255 degrees, but nylon has to print more slowly than other popular 3D printing filaments, so even small prints can take a long time. Whatever you do buy, make sure that the hot end (i.e. extruder and carriage, etc) is all metal and can withstand the heat. Some 3D printers had plastic parts in their hot ends and the printing process would slowly melt and deform the carriage. Mine did, but I have heavily modified it with lots of aftermarket metal parts.
Another highly useful feature is a heated bed. I would highly recommend that whatever printer you buy has this feature. It greatly increases the adhesion of the plastic to the build plate and allows you (or at least it allowed me) to print large flat structures without warping. Key for electronics enclosures!
r.b.