Bikerdude,
I cannot add much more to the excellent advice offered in some of the previous posts.
My experience - I had riden small displacement metric bikes for about 7-8 years. I then remained dismounted for 25 years. When I got a 1200 sportster last summer, it was almost my first scoot.
To address your questions based on my experience:
1. There are better ‘starter’ bikes than anything HD makes. There is not much difference between riding a 883 or a 1200 in terms of the necessary skills and abilities. Both are serious motor vehicles. You are a rider with < 1yr experience on smaller bikes; you are very wise to seek out advice before moving to something bigger. Therefore you probably have a fair amount of mental maturity. If you feel comfortable on a sportster, then go for it with boldness and confidence. (Note: Take you brains on every ride. Leave any road rage and competitive inclinations behind. Ride it not like you stole it, but like your grandma would have to fix it.)
2. The 883 is not a ‘starter’ bike, but they can very easily be upped in displacement with conversion kits. Many have. The result is identical to a production 1200. (BTW, does anyone know why H-D charges such a huge price difference between comparable 883s and 1200s? The cylinder bore, pistons and spockets are the main components involved and I believe the manufacturing costs for labor, materials, etc. are almost the same. Someone, please tell me if I’m wrong here, but I smell 'rip off'. For financial considerations, I would recommend searching out the best deal on an 883, then for only a few hundred $ have the cylinders bores out, add new pistons, maybe re-gear, and ride away on a sleeper 1200.)
3. See 1 above.
Also I cannot refrain from adding the following safety advice. With any sporty use caution in low speed parking lot maneuvers as you can capsize easily. (Crash bars are a good idea.) The throttle return spring is way too light. Stay off those leaves in the Fall. Never trust a cage.
Please give us a final update on what you decide.