Well the easiest and least expensive is exhaust, ECU upgrade, and air cleaner. The bottom line is how much you want to spend, and what your final goal is.
Definitely the first step.
What follows depends on your goals and budget. At what rpm range do you ride? Do you want low or mid-range torque? How important is reliability and proven results compared to pushing the envelope? For example. 103s have had a lot less issues than 110s and can generate some pretty impressive results.
After you ride a while, you might consider suspension upgrades as well as engine work. After you get the torque power band where you want it, the difference between 95 and 105 HP probably doesn't make a lot of difference. But, unless you match the 175 lb design basis, you will definitely noticed upgraded suspension over the stock H/D stuff, especially the limp front end.
Good luck on getting your ride set up to suit your needs.