If layouts aren't you strong suit, try and find a sign shop that could use your services and team up with them. They can do your layouts and you can help extend their services offered, win win.
The shop I worked in Kansas City had a relationship with a guy just like you. We did layouts for his neon and channel letters and cut vinyl for him as well. We used his installation services when a bucket truck was required or we need installation help because we were too busy.
If you really want to try this yourself, I would use Corel. Corel is a lot cheaper and, in my opinion, easier to learn and has a larger image area to work with (150' x 150'). I have always preferred to draw and design full size if possible because it is easier and you can avoid production scaling issues.
If you do go down that route go to
macromonster.com and get
Shelby's AutoSize. It will save you a lot of time scaling your images. All you have to do is import your picture, draw a line on the image from the points where you took real world measurements, shift select the image, launch the macro, type in the dimension and it will scale and straighten your image. Now your ready to place your layout/design on the building for review and permitting. When you need to put dimension lines on your drawing you can use Corel's built in dimension tools or
GDG Easy Dim & Sample from macromonster.com.
Good Luck