Welcome to the madness!
My recommendation would be to go with the 7x18 instead of the 8.5 for these reasons:
1. Easier to tow; you can see around it better; the 8.5 you would need to have extended mirrors on tow vehicle.
2. The 8.5 would cause more drag when towing as it will stick out on both sides of your tow vehicle. I would imagine your tow vehicle MPG would be less than towing the 7x18.
3. IMO, 7x18 would be easier to get in/out of gas stations, parking lots, etc. I have a 7x16 and tend to use truck stops to fuel at because you will typically have more room for getting in/out than regular gas stations. Personally, I wouldn't want a wider trailer just for this aspect....getting in/out of gas stations, parking lots, etc..
4. For some reason, we tend to travel where many rural roads are only a lane and a half wide in places, ha! I would not want to have a wider trailer just for that reason, ha! I think our GPS knows when we are towing and likes to get us on those type of roads, ha!
Best wishes on your upcoming build; look forward to updates on what you build.
Harry