Please share more about Surcouf's asymetrical hull! I did not know this, fascinating! Why?
If the model is for static display only, then 1/96.
If the model is for R/C, then I might opt for 1/60 scale.
Surcouf was a unique and grand vessel, so some impression of size would be desirable. 1/48 is too big, needs to fit in that Honda. Even in 1/60 scale, this would still be a challenge, but very possible.
In any scale, every detail like AAA guns and deck fittings would likely all need to be fabricated for this model too. No Tamiya 37cm AA guns to kit bash here in this case as for 1/32-1/35 scale U-boats. The French AA guns master patterns would have to be all scratch built, so in one sense this free's up choice of scale. Would your customers likely be European (metric scales) or American (English scales)?
Consider cost and ease of manufacture, a larger model would be a put out of a large chunk of change money wise. Yet too small a scale, would see just as much work producing with less return in profit? Cost of materials? Epoxy?
Finally, I would ask very definitely those modelers around the world that have actually built a Surcouf in the various scales. Pay close attention to their opinions most! Did they wish they in hindsight had built bigger or smaller? Would a slightly larger model or smaller model hull have afforded a more efficient propulsion or ballast arrangement? Would a smaller model have made it a better regatta course runner if that was important to them? Or would they have wished a larger size to fit in some special effects for those 8" pop guns!
Would curtain size 'off the shelf' piston tanks work best, or would the model require because of size, a larger scratch built piston tank if the builder selected that kind of ballast system? A special sized tank might be more than a modeler might wish to tackle. Yet, these models are also challenging to begin with, other wise consider stamp collecting! Consider cost to the consumer and the manufacturer, and how much the consumer will still have to put out after the hull is purchased only too.
To summarize, I think 1/60 would be a good grand size, but only if it was best considering all the internal requirements, 1 or 2 piston tank, or a large WTC? What size WTC? Would an already commercially built WTC work best, or would a special one need to be made by the model builder. Try to select a scale (size) that affords the model builder the least amount of hurdles engineering wise. Keep it simple. But vitally, contact those whom have already built a large Surcouf model, there is no substitute for experience in what to build and what to avoid.
In my building program personally, a Surcouf would not be included. I have already selected my 'majestic five' for a life time of building goals. However one of these five is a French design!
(My Majestic Five)
1. British Trenchant SSN 1/64
2. German built 'Dolphin' SSK 1/43
3. French Arethuse class hunter killer 1/40
4. American L-class (AL-4 - WW1 vintage) 1/32
5. German Type XVIIB 1/32
My humble opinion