Get in a heating and cooling person. They should be able to help you get ventilation in, sounds like you need it. Depending on what you need to get installed that might include a bit of roofing if it needs to vent out that way. It might be able to be vented out the side or something too. They will also be able to help with heating and cooling options. If the attic isn't insulated, get it insulated. Depending on your ceiling heights, you can install ceiling fans to help move air.

It's a safe bet that your attic living area is subject to significant radiant heat gain. A vent fan isn't an effective way of dealing with that. You'll need to insulate the underside of the roof deck and do so in a particular way. I suggest maintaining a vented air space behind the slates incorporating both soffit and ridge vents. I'd install furring strips to create an air space, then install rigid foam insulation air sealed to the rafters and supplemented by spray foam to provide the required R-value for your climate zone.

If you plan to use the space for bedrooms, keep in mind that the window(s) in each bedroom will need to meet code requirements for emergency egress and the floor structure will need to meet code requirements for sleeping areas.

photos?

You probably need to have it be an unvented attic assembly, rather than a vented one. To get proper insulation into an attic often involves reducing head space, to get enough R value, which makes it go below code minimum height. Insufficient insulation also makes it not achieve code as well, but is more difficult to see or enforce.

Also need to be certain that the loading is adequate. Most attics are not framed (floor system/ceiling joists) with the idea tha this will be turned into living space. Unfortunately the time to investigate this and plan it should have been before you remodeled the lower levels as you may need to rip the roof and second floor ceiling to get to the mandated code compliance and make the "attic" safe.

"You probably need to have it be an unvented attic assembly, rather than a vented one." The converted attic space will need to have insulation in the sloped portions under the sheathing/skip sheathing. While ventilation isn't always required, for example, when spray foam insulation is applied directly to the underside of roof sheathing, my guess is that there is nothing between the slates and the sheathing. In that case, you'd need to allow for drying to the interior or risk rotting the sheathing. A vent channel the full width of the rafter bays does that. It's the most forgiving assembly and it allows for an easier replacement of the slates in the event they are installed on skip sheathing versus spray foam applied to the underside of the skip sheathing and slates.

Not enough information. We need the following:

- a section drawing through the attic space showing the span, size and spacing of the floor joists as well as the size and spacing of the rafters and the ceiling heights.

- a floor plan showing the dimension of the spaces.

- ceiling heights where different from what is shown on the section.

To save time and effort you could start with the joist and rafter information in case they're not adequate for the intended loads.

You will need all of the above listed information when you apply for a permit.