Congratulations on the purchase of your Series 3.
Do you have a performance concern with the stock setup?
The Solex (aka SLOWlex) was not the first choice of the engine designers, but because of the relationship that Rootes had with Solex at the time, it was a management decision to override the engineer’s preference for Weber.
In your case it may be the availability of the right manifold to help you make your decision as to what Weber setup you may want to pursue. As for single or dual, I’ll let someone else with that experience comment. I have a Series 3 too, and the single Weber has been fine for me. Balancing two carbs is slightly more work initially, but once its done there’s very little to maintain.