It's obvious there are 2 categories of mod's, Functional and Decorative. I lean toward the functional mods, especially if you're tight on funds. Here's what I have done, all of them "necessary", at least that's what I told my wife
Functional:
Sundowner Seat (we ride 2 up 60% of the time)
Baseplate, luggage rack and backrest
Pipes, on my second set, started to V-Mods then went with SE V-Mods. V-Mods a little too loud for long cruises.
SE Air Filter
Race Tuner (to dial in the pipes and filter, the HD flash works well too)
HD 28T pulley and belt
Sport wind screen (sold the Super Sport that came with the kit)
Wired in battery tender and 12 volt supply for intercom (Autocomm)
Backoff chrome lighted license plate frame (decorative and functional)
T-Bag roll bag for around town
SAC + half SAC tail bags for longer trips
~$2000
Decorative:
Gold key kit (came with the bike, package deal)
Stripped and polished the top belt guard
Removed lower belt guard
Swing arm bolt covers (satin)
Kuryakyn chrome mirrors
~$150 (not including gold key kit)
Mark