Hi!
Twin turboprops need only one.
Yes, but with high requirements regarding the flying experience of the pilot. In my part of the world (don't know how the UK CAA is handling this), the number of flying hours required is stated individually in the OM-A of each commercial operator and can be "negotiated" to some extent with the authority. For a start-up business it will be quite difficult to find pilots who meet the requirements. Additionally, single pilot operation is penalised by the duty time regulations and mostly incompatible with executive operations (A to B in the early morning, B to A in the late evening...). Anyway, most executive customers don't like single pilot operation, some require two pilots always and some even require yearly simulator refresher courses for all crewmembers!
Regarding piston twins: Stay far, far, far away from them (for commercial operartion). I have co-owned a charter business for ten years pre JAR(EU)-OPS operating a fleet of up to six aeroplanes (C421, C404, C340) and it was never really profitable. The highest cost factors were maintenance and AVGAS. The aeroplanes have aged another 10 years since then, hourly rates of mechanics have more than doubled, availability of spare parts can be very problematic. The price of Avgas has risen a lot and is becoming more and more difficult to find.
So I personally would not start with anything other than a modern - still-in-production - turboprop. The choice is very limited and prices are high.
Greetings, Max