The Auto Meter a/f guage taps directly into the stock oxygen sensor signal, and HKS doesn't make one. On a forced induction car it's nice to have as a warning light against leaning out when you're dropping the hammer but that's pretty much it. Pretty much any dyno has a wideband oxygen sensor which will give you a plot of your a/f ratios during the whole dyno run which is much more useful than looking at some blinking lights running off the stock oxygen sensor.

The S-AFC is a nice piggyback, if you had a different one in mind pretty much any one will work. What it allows you to do is tune the fuel map. A reprogrammed ECU generally will have a different fuel map, timing map and rev limiter from stock. You can get a Hondata s100 for $245 (that's cheaper than the S-AFC) which fits inside the ECU and lets you tune fuel and ignition maps, plus the rev limiter. So all you'd have to do is go to a shop that sells Hondata and ask for an s100 and dyno tuning. For the price you can't beat it.

As far as any more boltons to grab before going to the dyno, I'd skip the throttle. It's not really a restriction so opening it up isn't really going to get you more power. You could get a skunk2 intake manifold, that should be good for some power.