There are many considerations for this task and here are a few observations I would ponder while planning a similar job.
First off, I print decent sizes from 135 format images and I have shot for artists art books, most of the time they are OK but in many conditions a medium format capture will wipe the floor of 135 with regards detail, tonality and DR, even in modest sized prints, especially in challenging light (like early mornings). But it can be very subject and style dependent, without knowing the post processors style it is impossible to say whether MF is a better approach for this specific image.
Second, if you are shooting at Varanasi from a boat stitching isn't a practical option. I have done it.
Third, if you intend to use shift, nodal/entry point stitching is not an option, well I have never managed it, all I ever got was some weird projection anomalies.
Fourth, if you intend to do a traditional stitch with a 135 format TS-E 24 to get a 35mm fov then you'd need to use a 2XTC, and whilst the TS-E 24 is a sublime lens you cannot fail to take an IQ hit. If "Best Possible IQ" truthfully is the goal that is not the way to get it.
Fifth, MF is not that steep a learning curve, you will need a laptop with lots of RAM and HDD space, and shooting tethered really helps. If you have methodically shot landscapes etc then there are no real gottchas to worry about.
Sixth, shooting MF opens up a plethora of exotic very high quality lenses that will give you 80MP of detail and massive DR in one shot.
Seven, if you do go the 135 format route the best way to shoot for a 2:1 aspect ratio is a TS-E 24 + 1.4 TC and do a four shift "rotation" stitch. That is four diagonally shifted images, this method gives good overlap and enough cropping distance to lose the corners closest to the image circle.
Eight, email Roger over at LensRentals, he loves a challenge like this and will tell you straight what body, lens and software combo would be most effective.
These are just a few things I'd mull over in preparation. As a first thought, with "Best Possible IQ" in the brief, it would take an awful lot to make me drop rented MF to even consider 135 (and I am a huge 135 format fan), the differences are not just pixel numbers, but AA filters, DR and tonality.
Thank you so much. So much valuable information. You are the rare one who says medium format will look better even in small size prints.
We intend to get a sturdy crane which will go out about 10 ft into the river and improve the angle of photo. I am not looking at a stitch option, the job requires several photos taken at quick succession to later remove people and create an 'empty' shot.
When we rent stuff like MF in Bombay it comes with a guy who knows the camera technically.