No problem. Some of these things are specific to the Fractal Mesify C case I put in my build list (aka, what size, where, and how many fans to add) and some are more generic.
Generic:
You generally want more air being forced into a case than out of it (also referred to as positive pressure). This forces the most possible air through fan filters (if your case has them) to keep dust buildup inside the case to a minimum. If there's more air being forced out of a case than into it, the air is going to come in through all the openings to balance the airflow out, many of such openings may be unfiltered.
AIO (water cooling) radiators are VERY airflow restrictive because their fins are so close together. Because of this, it's advised to use fans with high "static pressure" (aka how much Pa of air pressure a fan can produce) to give the fan(s) the best chance at getting air though the rad. This is a very easy phenomena to observe (even with high static pressure fans installed). Simply crank the rad fans to 100% and put your hand (or a tissue) in front of the fan. There will be a noticeable amount of air splash-back (my term). Because of this, obviously the full rated CFM of the fan(s) is not passing through the radiator. Regardless of where the AIO is (intake or exhaust) this needs to be accounted for when you're doing your CFM math (I generally count a fan on a rad as 1/2 it's rated CFM). As explained above, this is more important if the AIO is set as an air intake (since you may not be getting as much air forced into your case as you'd thought)
Bigger fans move more air at the same rpm (obvious), so if there's mounting holes for multiple size fans in a location, the largest one is generally preferable as long as physical clearance to other components isn't an issue. Lower rpm = less static pressure though also (if you're replacing/adding to the stock fans that come with an AIO, or having to put fans in the front of a case that doesn't provide ample ventilation for the front intake fans).
Generally there's a point where you reach diminishing cooling returns by adding more and more fans. Also each fan makes noise. Therefore, if you add 5 more fans (and their respective noise and cost) but only gain another -3C on your CPU and/or GPU...is it worth it?.....Different people have different stances on this. My personal rig is very quiet and even while gaming it's noise output doesn't/barely increases.
Specific:
Having specc'd the Fractal Meshify C and a H115i (280mm AIO), the H115i has to go in the front (per manufacturer spec page). That's going to be set as air intake into the case. However, as explained above, the airflow is rather restricted, so adding an addition intake fan in the bottom 120mm filtered fan mount location will push fresh cold air right at the GPU. That stock 120mm exhaust fan roughly cancels out the bottom 120mm intake, so we've still got positive air pressure from the (1 equivalent 140mm fan) AIO fans. Therefore, a top mounted, low rpm 140mm fan could be helpful. I'd put this as close to the AIO as possible to suck that heated air out of the case before it gets to the GPU.
As to what fan(s) to buy.
That's going to be pretty contestable since everyone has their own opinions and experiences. Noctua fans are high quality, but they're expensive, and other manufacturers make high quality fans also. As a general rule of thumb, anything under 1300rpm isn't going to make much noise. RGB fans usually sacrifice performance for looks.
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