This is right up my alley. I am so sorry I didn't get back to you before this, but I had a loss in my family a few days before you posted this and I am only now getting back into online life... let me see if I can help you.
PUNCHLINE FIRST: Skip to the bottom if you want a nearly-free way to start doing this that will work on your iPad or iPhone immediately. But if you do want a controller, read on.
My primary recommendation for you in this area is to go with the "industry standard" when it comes to grids of buttons. As you may have seen from your research, which I can imagine would truly be nightmarish for a beginner, there are two basic kinds of pad grids:
- the 16-pad 4x4 array of larger buttons, commonly referred to as the MPC format since the Akai MPC drum machines were where it got its start, and
- the 64-pad 8x8 array of smaller buttons. Real music tech geeks will refer to this as the monome array (since that's where it was invented), but most people call it the Launchpad array after the Novation Launchpad and its many children.
For what you're doing, I strongly recommend the Launchpads. They're light, affordable, rugged, easy to set up, and fairly seamless to integrate with software that will do precisely what you need -- a simple version of which comes included with them for your Mac or PC, and a nearly-free version of which can run on your iPhone or iPad.
First, the hardware: Novation just released the newest versions of the Launchpad and Launchpad Mini, called the Launchpad X
and the Launchpad Mini mk3
These offer 64 pads with RGB coloring so you can sort them visually, and each button can launch a sample, loop, or MIDI clip. They also can serve as mixer controllers to turn sounds up or down, add effects, etc.
The main difference between the two controllers is size and performance features. The Mini is a lot smaller than the X, and its buttons aren't velocity or pressure sensitive. But nearly everything else about them is identical. They're powered over USB, and if you hook them up to the right software, configuration will either be (a) super-easy or (b) done for you.
Which software? Both come with a license for Ableton Live Lite for Mac or PC. Ableton Live is fabulous for what you want to do, because you can lay out samples and loops in a grid on your laptop screen, color code them, edit them, label them, and the Launchpad will trigger them for you. You can set them to loop or play once, cut off when you let go of the button or finish playing on their own, and with a couple of hoops to jump through, you can connect Live to other software to share audio, or export Live Projects for use elsewhere. For your needs, Live Lite will probably do all you want for a good long while.
On the iOS side, Novation's software division Ampify makes a Launchpad app. It's free to start with, but like many such apps, they get you with in-app purchases. Still, for not too many dollars, you can buy access to the full palette of built-in effects and to the ability to import your own samples. The main place they get you is with music loop packs, which cost just a couple of dollars each but are as addictive as potato chips.
Launchpad has a 6 x 8 grid of pads and a 2 x 8 grid of effects triggers. It's like the grid Session View of Live, stripped to the bare bones, but you might still find it enough for your SFX-triggering needs. Oh, and now we get to the punchline: it is a lot more fun to play with a Launchpad controller, but it's perfectly usable all by itself and controllable from the touchscreen with pretty much all the functionality of the hardware.
I hope this helps.