I've done only 1 spacebucket grow as I loved experience so much so that I bought a tent right after XD. I grew mine in coco as well and based on my experience, your component looks better than mine, especially those fans as I got away with 2 cheap CPU fans for intake/exhaust.

I haven't experienced with the line of nute you provided but I had success using GH flora trio and calmag, which should be similar to those. I measured ph using droplets that change color based on ph level. These are not as accurate as some electronic ph testers but since these have to be frequently calibrated, it was too much hassle to me and using droplet works perfectly fine for me even today.

I think you might be over thinking a bit, but sounds like you have done a good amount of research and ready to grow a plant in space bucket. Below are quick answers (my personal opnions) to your questions. I could be wrong here and there but I do not think it would make any significant difference.

- Do I need an additional fan for my exhaust, or can I connect that filter directly to the top of my bucket?

If you are growing single plant, i think your fan setup is more than adequate.

- Do I need to purchase this 3 product bundle or can I get away with just one of these (or another product? - UK based if that helps for recommendations regarding nutrients) (If just one of these, which one is best all round for my grow?). I've also seen people talk about needing to add CalMag in with these products, how much and how often would one want to be doing that? Final nutrient question, when do I start giving nutrients?

You are growing in coco so you need to feed from the beginning, as coco is a hydroponic medium that does not have ability to store nutrient. Try looking up a good feed schedule and sticking to it to make it easier for your first try. ( i would add calmag to your nute list).

- Do I need to purchase a lux meter? I'm sort of getting to the end of my budget range (was hoping to keep things under £200 but I'm a little over that at the moment) and if I can get away without it I think I'd want to hold off buying one until my next plant

This sounds like an overkill, you probably won't need it.

- Do I need to purchase a water pH tester AND a soil pH tester? Are there combination tools you can buy so I don't have to purchase 2 separate ones?

I use cheap droplet like the link below. I dont measure soil ph, just ph the nute solution.

https://www.amazon.com/General-Hydroponics-pH-Control-Kit/dp/B000BNKWZY

- Do my fans need to be running 100% of the time or should they be on a timer with the lights?

Yup, i would never turn the fans off as girls always need ventilation.

- Will I need to use a humidifier?

I do not think it is necessary for spacebucket.

- Finally, will the filter I'm planning to use be enough for a high degree of stealth if I'm just growing one plant? I live on my own but I'm a bit paranoid about it all so wanting to get as close to 100% elimination of odour as possible. On that note, is there any likelihood of raising red flags with the electrical company running this UFO light and the filter for 16-24 hours per day?

If all exhausted air goes into carbon filter, it should provide a very effective stealth level. And don't worry about electricity bill. There are millions of people who would have small spike in electricity bill and utility probably cannot care less of your small spike :)