First off, welcome to the forum...
2nd/ have you rode anything? Do you have a motorcycle license? If you do I would suggest a demo event to check out several different models.
3RD / I like to looks of the RK but I could stand not having a radio..love the tunes going down the highway.
4TH / any passenger? if so, make sure she's comfortable with whatever you choose. If Momma ain't happy, no body's happy.
Bigger is always better
Stage 1, quick answer, it's free flowing exhaust, better air filter and a remap of the fuel management system, wakes it up a little.
Exhaust systems, mufflers etc are very subjective, what someone else like, you may not...best to go to a bike night or dealership on a weekend and listen to other bikes, ask what they are running. Me, I like more of a quite muffler. The Screaming Eagle touring fit me fine.
Again, very subjective...gonna have to look around and check out a bunch of different bikes, ask first before sitting on any one's bike..
There's a ton of seat makers out the, inter search should led you to a bunch of different ones, C&C, Corbin, etc...might even check out a horse saddle maker....or upholstery shop.
Yes, big investment, go slowly, ask around...walk around and check out all you can. Your money, your butt, your bike! Don't buy what someone else likes, get what you like. If the RK is your bike, go for it...
Ask yourself, "what am I going to do with the bike?" " how am I going to ride it, around town, trips?"
Good luck on your choice, I love motorcycles period..all kinds, all brands...The RK is sweet, but for me the Ultra is what I would choose, but again, that's me.