Make sure your area will allow you to do a drain/dump in your garage if you decide to go that route. We are building our building too right now and the county will not allow it, despite us being on septic. I think they're worried about people draining stuff into the storm water ditches that run in the front or something... who knows, stupid, but it is what it is. But we were looking at homes in Boise last year and there were quite a few attached RV garages that had a dump in the garage. Those were all on city sewer systems and were added at time of home construction though so maybe that's the difference. That was a nice selling feature when we were looking. So, I think if you can do it without it costing a crazy amount, it would be a nice thing to have for both you and any future owners.
We are running 30A into ours, thankfully the electrical box and a 30A outlet are already on that side of the house so we just have to run it from there. This is less of an issue for you in AZ, but we are also running a gas line so we can have a heater inside. We had the line put in when installing landscaping and a fire pit so we only have to run it another 20' or so to get it into the building. No AC either but ours is stick frame with siding etc., so eventually we plan on adding insulation/drywall which should help in the summer. It made a big difference in our attached garage in terms of keeping it warmer in winter and cooler in summer.
Ours is also not as large as yours, 30x50 for us as that's the max size our HOA will allow. I think I'd rather have more width than length if you've got the room though, since if it's wide, there's less chance of stuff accumulating in front of other stuff, making it hard to get it out. Since we are more limited on size, we at least made sure we could fit the truck and trailer hooked up in front of the building, but behind our fence, so we could hook it up the night before a trip but not have it sitting in front of the house or in the main driveway.
