From what I've read, the Ipac encoder has 2 joystick inputs, 8 buttons, and 5 other inputs (coin1/2, start 1/2, shift for another complete set).
You're basically correct except it should end "... and 4 other inputs, and any of these 28 buttons can be assigned to also act as shift for another..."
Technically, OTOH, Ipac has 28 keys + 27 virtual (shift) keys. The non-shift default settings match mame's default player 1 & 2 joysticks and 8 buttons and start and coin, but you can change that both in mame and in the ipac. IOW, you could for example use the Ipac for 4 players, each with only a joystick and 3 buttons, or whatever you want. If this confuses you or you know you're only going to do two players, just ignore this paragraph.

i was originally going to go with Happ Supers with an Ipac on a street fighter config, then a poster said to just get the Ultimarc U 360 which has 8 button inputs for it included.
So I'd like to know, can I get away with just buying the U360, or is it better to get the Ipac and Happ Supers.
Personally, I really like the u360s, the supers are not bad either. However, some people like to feel the corners, which you can't with supers, and need to get the 4-8way restrictor on the u360 to feel.
Are you only going to mame, or are you going to play other emus or PC software? And what games do you like in mame? Are you fixed on one type, or like many different types?
The u360 has many pluses:
works with any game that reads joysticks (ipac works as a keyboard)
different settable modes, including analog, 8-way, 4-way, 4-way diagonal, and 2-way
can have circle, square or diamond restrictor plates.
can use ball or bat handle
has 7 additional shift buttons for a total of 15
mmore flexible than supers and most other joysticks
The only minus is:
only 8 action buttons per player (the 7 virtual shift buttons aren't fast enough fro game play).