I don't have an answer for #1 or 2. The horn issue is probably rust in the steering column preventing a good ground, so you have an intermittent beep then nothing. I have a repair video for this.

My driver's seat belt light would come on and stay on from time to time. It finally came on permanently. Thankfully it didn't beep all that time. There is a connector at the base of the B pillar retractor where the seat belt wire connects. You can either replace the retractor or just disconnect the wire and the light goes out. That's what I did.

The TPMS works with tire rotation. Somehow the system compares the rotation of the front tires and if they don't align the light comes on. There is a button at the bottom left of the instrument panel to reset it. It tells you how in the owner's manual. I'm not sure but I think you have the motor off, then press the button while you turn the key to "on" (idiot lights on) but not start the engine. The system should reset and the light goes out. If not, then there's probably a bad sensor on one of the front wheels. Check owner's manual to verify my instructions

 

there’s anything else I should replace at the same time?

Could save :30 by avoiding removing the vent twice, if you plan to replace the timing belt anyway.
I covered my tpms light with black electrical tape. Does your abs work? It's what determines the tpms on my 99. Gonna fix that someday.🤣

the two tone paint is actually in very good shape!

I agree, my lower 1/2 wasn't painted so the plastic looked like crap.

 

I covered my tpms light with black electrical tape. Does your abs work? It's what determines the tpms on my 99. Gonna fix that someday.🤣

I agree, my lower 1/2 wasn't painted so the plastic looked like crap.

On our '99 we've not had many problems with the TPMS. There were some years where it would keep coming on and going off, especially in cold, rainy weather. I bought new tires for the front a couple of years ago and we've not had the problem since. Either the owners manual or I read somewhere online that somehow it compares the number of rotations to both wheels and if one is constantly different it thinks one of the tires is low. I always thought there were sensors on the spindles that performed that job. In any event, it's not very accurate but my light doesn't stay on. If it does I check the then perform a reset to turn off the light. I monitor the tire pressure with the change of seasons and air up as necessary.

 

TPMS must have been mandated or something. Silly things, whether rotational or sensors, give false readings more often than not. Another expense.
I have two diesel air compressors, The England one is over fifty and uses pneumatics for controls, never fails... maybe a leak taking 5 minutes to find and repair.
The Japanese one uses electronic controls, designed to be idiot proof, always fails. Nothing broken just refuses to run til the idiot proofing is diagnosed and repaired... if you can figure it out and find parts. That's how I bought a $20,000 machine for $2000.

I thought about doing the coil packs, but they’re a bit pricey.

There's only three on my 99. Easy to replace anytime. No reason to preempt failure.

 

1. My valve cover gasket is leaking. I already have the gaskets to replace it. My question is, is there anything special I should know, and if there’s anything else I should replace at the same time? I also have spark plugs, so that’ll be a given. I thought about doing the coil packs, but they’re a bit pricey. I heard that Denso made Toyota parts, so if I do replace them, is that a reasonable brand to go with? Edit: it was NGK coil packs I found, not Denso.

You would also need to replace Spark Plug Tube Seals, if they are not already included in the kit you have, and Air Surge Tank To Intake Manifold Gasket.
Since you are going to have easy access I'd also replace PCV valve.
Don't forget to get Toyota Seal Packing 103 FIPG Sealer or equivalent to re-seal half-moons.
Also, valve cover might be leaking because it is not clamped down tight enough anymore. This sandwich of a washer, as you can see from the pic, just gets deformed over time and no longer provides proper force. Also, bolt design won't allow it to just be tightened more. So it would be good idea to get replacement washers alone or bolt with the washer. As a ghetto solution there are few vids on YT where people would reuse existing washer placing a regular washer on top of it to compensate for that permanent deformation.

 

1. My valve cover gasket is leaking. I already have the gaskets to replace it. My question is, is there anything special I should know

The front valve cover is pretty easy. The rear one requires that you remove the intake manifold. That's no small task. I replaced the front valve cover gasket and spark plug tube gaskets myself and had a shop do the back one. Whether you're doing it yourself or having a shop do it, replace the spark plugs at the same time.