Hi Ravs,
Welcome to the hobby, it sounds like you have an exciting build in store and many years of joy and strife to come. Look forward to following along if you go the tank journal route!
Certainly not an expert here, so will defer to others on most points. But a couple of thoughts on your kit and journey to come.
It seems to me from my short time in the hobby and from lurking on various boards that when problems arise there are usually multiple explanations for why, and the good will of the forum always comes up against the complexity of the hobby/the difficulty of not "being there." Hence the interest in as much detail/specificity as possible about the tank setup when diagnosing next steps and so on. That can lead to a kind of endless cycle of testing and upgrading, gear acquisition and what not. So it'll be interesting to figure out what your own philosophy is, how you draw on your DIY experience, while integrating new lessons, and so on!
On that note, I think your idea of getting a quarantine tank is a good one. Maybe hold off on giving it to the son until you've set up the tank to your satisfaction. You could get a lot of use early on treating plants from your LFS to remove pest snails/planaria before adding them to the tank. This would be a way of having a "clean" setup without having to rely on tissue culture plants, which can be sensitive. You could use it to transition emersed growth plants to immersed with a bit more control. You could also use it to acclimate livestock comfortably, calibrating them to the water parameters of your main tank. And then there would be the uses as strict quarantine tank in case of issues with fish (the parasite ich comes to mind as one frequent example).
On the dosing pump side, I don't know that I'd consider this a necessary piece of kit. It depends how deep you want to get into customizing different fertilizers and additives, and that will in turn depend on the kind of habitat you want to set up, how you want to approach taking care of plants and livestock. I'd second the suggestion of going with the NilocG premix solutions. They're affordable, and either one would be easily dosed with precision. Based on readings here and elsewhere, I went with the PPM mix to give the fish an easier go of things and because of medium intensity lighting.
Final thought, have you got any water testing kits? Those seem like a useful additional to the early set up. Like everything, these come at different price points with varying degrees of precision. Search the forum for some chatter about that. Salifert, Sera, and Red Sea are highly rated. Technically, you could see if the tank cycled just from an ammonia test (see the 2hr aquarist website for cycling, which is also a whole world of debate), but in the long run you're going to want to test more than that. Nitrite tests are good for cycling, but also for detecting spikes (e.g. in case of hidden fish death/increased bioload). PH/KH/GH are good to know for figuring out stocking, and then here for troubleshooting issues. These things can get pricey. If you know you're going to want to helicopter parent everything then it would make more sense to invest in a full kit from the get go (there's a sera master test kit at $100 that would probably do the trick). If you're not sure, then go with test strips that cover the basics and then you might add in individual tests as you go and you get a sense of your needs.
Good luck!