Roofs are difficult places to install solar panels, so they're not my first choice.* There are several issues when you put a panel on a roof, and they have to do with fireman access to the roof.
You need to arrange aisle-ways between rows of panels so that firemen can reach the roof. This will effectively break the solar panels into sections.
You need rapid shutdown: an ability to de-energize the solar panels from a switch on the side of the building.
- If you want to pump up a series string to multi-hundred volts, to reduce voltage drop, then you need a set of relays that will disconnect each section of panel such that the inter-section voltage is less than 30 volts.
- Voltage within each section must be less than 80 volts.
Your PV array also must be listed or labeled for rapid shutdown, i.e. made for that.
One way around the rapid shutdown issue is run the array at less than 30 volts. This would require flowing 300 amps in total. So you'd want something to carry that kind of current. The obvious thing to me would be aluminum panel racking.
As far as roofs, you haven't discussed whether you are building a generic common roof just like every roof in the past 50 years, or whether you are engineering the roof specifically to support solar panels. For instance I would design it with parapets to attach solar panel mountings to, without necessitating roof penetrations. This would also reduce the roof to a series of shallow trenches, which would lend themselves to rubber roofing material, making it easy to maintain in segments.
Roof penetrations are generally bad, and create opportunities for leaks. So you should avoid them whenever possible. Despite the sunk cost of having the electrician run the conduit to the attic, I would not follow through and come through the roof; I'd route over the eaves and enter the house beneath an eave.
Given all these roof hassles, unless you have factored for all the above considerations, you may want to find a place other than the roof. Surely you have a driveway, parking spot or patio you'd like shaded?
If you're committed to doing a plain old roof, then have roofers do it. Then have a solar installation specialist install the solar panels.
Lastly, be careful about service panel sizing. Sizing is not an issue if your panel's bus ampacity is large enough to handle your main breaker trip and also your solar system output (9kw = 40A).
Speaking of sizing, panel space becomes a factor especially if you start doing power-down recovery, Tesla Powerwalls and the like. Most builder specify a service panel in the 20-30 space range. Tell him you want a 40 or 42 space panel. It also helps to use a panel type that supports a wide variety of additional features - Square D QO or Siemens come to mind.
* Actually, my preferred location is Antelope Valley. It's near L.A. which has a huge commitment to solar, and huge peak demand with little alternative, so they pay top dollar for solar peaking power. Take that money home and buy any old local power.