9t9r

Hello,  i think the seq v4 software is already capable of doing all that.  check out its remote control functions and midi implementation.

As regards hardware, you can go any way. Lots of commercial devices offer pads and sliders, just program  them to control the SEQ . If you want a quick and relatively standard solution, buy an existing controller for sure: a Kord Pad Kontrol,  a Launchpad, a Mackie, a  Presonus fader port, a stein berg cmc or cc121, a keyboard controller with some pads & sliders. So many options out there...

DIY is for the love of building and  / or if you want something extraordinary -like the unbeatable SEQ V4. It is definitely neither cheaper nor faster than ready-made hardware.  If you do want to build - NG is the way to go as Shruriken suggests.