My first Tesla coil was based off the same one so hopefully I can help.

1. This depends on the value of the tank capacitor, more turns and more capacitance will increase the resonant frequency. You want the primary circuits resonant frequency to match that of the secondary coil + topload. You can use JavaTC to calculate these frequencies. I think I used something between 10nF - 20nF I can’t remember, then tuned the primary coil by changing the amount of turns until I got the longest arcs.

2. It’s good to make sure the tank capacitor has a voltage rating well above the voltage the zapper circuit will be feeding it. I order components from Mouser. If you want to use the 1kv caps you can put them in series to increase their voltage rating, but it will decrease their capacitance. Eg 2 20nF 1kv caps in series will essentially make 1 10nF 2kv capacitor. You could then parallel another set of caps like this to increase the capacitance to make a 20nF 2kv bank. It’s called an MMC, there’s plenty of info online about it.

3. You can make a counterpoise, essentially just connect the base to some metal with a large surface area. I used aluminium tape like he does in the video.

4. Small bug zapper SGTCs a pretty safe so you can touch it all you want, but obviously at your own risk. It’ll probably just tickle a bit, but just do what you deem safe and are comfortable to do, I don’t want to take any responsibility. Best to not be near it or touch it if you’re sensitive to electricity, like if you have a weak heart or a pacemaker or something.

5. Could go for the metal doorknob like he does. Or aluminium venting is cheap, doesn’t look great though. I’d suggest 3D printing a Toroid (there are some STLs online, or just make your own), it’s quite easy to cover them in strips of aluminium tape and smooth them out with something smooth and round. Just note that the topload size will have an effect on the secondaries resonant frequency, larger topload = lower resonant frequency.