All the dandies have 2 double beds and will sit 3 on the settee. The designer and discovery have a single bed as well.
An awning can be fitted which will give more room.
In my designer I was able to store all the camping related items inside the dandy with awning on top. This was porta potti, water and waste containers, step, electric hookup lead, TV, cushions, sleeping bags and pillows, electric fire, cooking pots and pans, crockery cutlery etc.
We took a holdall each for clothing in the car which lived on the fixed bed by day. The bedding all went behind the settee when made up, the bedding for the fixed bed stayed on it.
We had a small chest fridge which fitted bedside the kitchen. This travelled in the car on 12 volt and ran on gas or mains electric on sitter. this stayed in place overnight as only a small part of the 2nd bed was lost.
The porta potti came in at night. It fitted by the door just to one side. I made up a curtain to separate it or you could use a toilet tent.
So I think you will have enough room in any of them although the designer and discovery have just that bit more. All are max load of 500kg so you should be able to tow it OK.
You certainly have the right reasons for a dandy and it wont disappoint on these. When you say blue dandy, do you mean the pale blue and white or the dark blue and white.
Your only problem as I see it is the weight of the extra kit you want to take like bikes etc, although bikes will travel very well on a roof rack on the car. This gives you the ability to drive to somewhere nice to ride them as well rather than only on the dandy.
You can get bike racks for dandies and some have made them.
The low towing limit may be due to the chassis construction rather than the engine power. What year and model is the fiesta.