I use ketamine quite a bit in my pediatric intensive care practice and have 30 years experience with it. It's a very useful drug in children for sedation and anesthesia, and sometimes for severe asthma. It has several well-known effects that you might find useful.
It always increases heart rate and often increases blood pressure. It makes the eyes quite red. It causes nystagmus (roving eyes). It causes an increase in respiratory secretions, sometimes dramatically so. These things are dose-dependant. The hallucinations are somewhat related to total dose, but they can happen at any dose. They are far less common in children than in adults, which is what really limits the use of ketamine in adults. I've seen the drug produce the equivalent of an acute psychotic episode in an adolescent that lasted for several days.
Regarding breathing, one of the advantages of ketamine compared to other anesthetics is that respiratory drive is well maintained -- people keep breathing even with quite massive doses.
There is no antidote to ketamine -- we just let it wear off. It is relatively short acting and its physiologic effects are gone in about an hour at most after an IV dose. That being said, many patients will feel a bit odd for an hour or two and can be emotionally labile and react strongly to suggestive stimuli. (I keep the lights and noise level down until they have completely emerged from the drug.)
To lessen the side effects we often combine ketamine with a drug to reduce the secretions (e.g. glycopyrrolate) and an anti-anxiety drug (e.g. midazolam). The latter markedly reduces (nearly eliminates them, actually, in children) the incidence of hallucinations at emergence from the drug.
Ketamine can be given IV, IM, intra-nasal, or oral. The onset of action of an IV dose is about 2 minutes, IM about twice that. The first sign that the drug is working is roving eyes (nystagmus). I always watch for that.
One thing you might find useful for your plot is that ketamine comes in 2 strengths -- 10 mg/mL and 100 mg/mL. I've seen several accidental overdoses from using the more concentrated solution and thinking it was the dilute one.
Usual dose IV is 1 mg/kg, IM 3-4 mg/kg
Let me know if you have specific questions I can answer