Hi Glen,
I'm one example of a guy who could only do narrowband for a number of years exactly because of light pollution, so such thread could potentially be quite useful for people starting off their adventure with (narrowband) astro
From my experience mount's capability to guide precisely is essential in narrowband, given that narrowband usually requires longer exposures than LRGB imaging, if we want to reveal finer details. This prerequisite perhaps can to a degree can be mitigated with low read noise CMOS cameras?
If we are on a very limited budget then we can assume a modestly performing mount that will put a cap of the length of useful exposure. Therefore photographic speed, perhaps some under-sampling to hide imperfect guiding and low RN are the key to good data. Speed can be achieved with a fast focal ratio (I'm taking extended object not stars), while low RN is a given with relatively affordable CMOS cameras.
If we assume that the sensor will be 3.8 microns as in ASI1600, then I propose a 500mm focal length or thereabouts, that will give some but not excessive under-sampling. If we would not like to go slower than say f/5-6, this in turn suggests aperture of around 100mm also meaning the scope could be carried by a modest mount. To sum up, one would like around 100mm f/5 telescopes to match with ASI1600 (or similiar). This leads us to FSQ 106...
But more seriously, given my very positive experience with TS ED doublet for narrowband, my suggestion is a TS-Optics ED 102mm f/7 doublet with a 0.8 TS reducer resulting in f/5.6 and 560mm focal length. This is what I used in the past with 3.69 micron pixels and got very satisfying results. The mounts were HEQ5 and later AZ-EQ6. The cost for this scope+reducer is about 1000 AUD plus shipping and import taxes.
A cheaper alternative would be 80mm TS ED f/7 doublet with the same 0.8 reducer for about 850 AUD plus shipping and import duties.
More saving could be made by buying a second-hand telescopes with similar parameters.