If it is for a bathroom, and you have long steamy showers in the modern style, you need a throughput of about 240 cubic metres per hour.
(For background, the cheap 4-inch fans often fitted by builders are about 80 metres per hour, and are adequate for a small WC cubicle)
You also need a run-on timer to continue drying the room after you have left it. The timers are usually adjustable but 20 minutes is common.
Some people have an aversion to ventilation and will not switch on extractors. IMO the best control is a fan that comes on automatically with the light switch. Some people prefer a PIR movement detector, though this may not work if you are wallowing motionless in a bath. Humidity sensors I find do not start up quickly enough, and do not react to odours.
Next you need low noise. One of the reasons people object to ventilation is that some extractors are quite noisy. Compare the dB rating of models you are looking at. As you are planning a loft installation, you can mount it on a thick ply base, padded with carpet or other material and screwed to the roof timbers with rubber washers under to screw heads to deaden vibration. It need not be on the ceiling.
I find the Soler & Palau "Silent" range very good. They have a ball-bearing motor which should run quietly for many years, mounted on rubber bushes, and the build quality seems very good. They are also sold under the "Envirovent" brand name (same company) and price and availability sometimes vary.
Example
Or a bigger one.
Incidentally, rigid duct is very superior to convoluted hose. It is more durable and less prone to trapping water, dust and fluff. Apart from the short riser through the ceiling, you should slope it slightly so any condensation runs down the straight tube and drips outside. You can insulate it to reduce condensation.
You will need a gap under the door so that replacement air can enter the bathroom.
