For plugs, ONLY ONLY ONLY use NGK OE Laser Iridiums. The part number you're looking for is LTR5BI-13. Not even kidding, don't even try to use a different plug than that. Yeah, they're expensive at about $12 each, but don't matter, it's the correct part for your car. Besides, when you think about the fact they are rated to last 100,000 miles, the price is far more palatable.
For the manual transmission fluid, use Redline MT-90. Again, expensive, but it's the right stuff.
For the A/C, switch the relay from the horn to the compressor in the fuse box under the hood. They are the same part, and right next to one another. You may get lucky and it's just a bad relay! Or, because your horn is sick sounding, both relays may actually be bad. I think there's actually three relays under there, and I *think* the third is the same as the horn and A/C, but verify that. The third relay, I can't remember what it controls.