New to water cooling

The Corsair H100 is a very good all in 1 unit. I am currently running an Antec 920 which is equivilant to the Corsair H70 / H80. It keeps my 2500 nice an cool. Saying that you have a switch 810 so the world is your oyster with water cooling. It really isn't that hard to get it setup (i have had custom water cooling in the past). It is especially nice when you can put dye in the res and make your case look the bomb. I have always used 1/2inch barbs with 7 1/16 inch tubing. The tubing is very tight on the barb but I don't like using the clamps as I think they look ugly. I work on the equation that you have 1x120mm radiator per watercooled device. If i were cooling a CPU and single GPU I would use a 240mm rad.

When you build a custom water cooling system with good components, it's something you'll have for a long time. It all depends what you want and how much involved you want to get, I always build water cooling system that have more cooling capacity than I actually need, this way if want to add more components (second GPU) later I don't need to worry if my WC system can handle it.

No, you cannot change anything on the H100 without completely defeating the purpose of an AIO like the H100. It's designed to be completely sealed, no need to replace anything, etc. The pumps on the H100 and similar are designed to be very minimal, able to work with its loop only, and nothing more.

Don't get the Rasa 750 kit, the pump in those are prone to premature failure, especially with multiblock loops like additional GPUs.

If you want to get a kit with good expandability, I would get this kit.

Otherwise, here's what I would suggest for you with a Switch 810, from jab-tech.com (they have pretty good prices).
XSPC Raystorm
XSPC EX420 3x140 radiator
Swiftech MCP655
If you want reservoir/pump in one unit, get either the tank reservoir or bay reservoir.
Otherwise, choose any one of these reservoirs. Whatever suits you best. There's a lot more options from other resellers though like FrozenCPU.com and performance-pcs.com
Tubing: I've had good experiences with Primochill tubing, and the price is really good. Choose the size which suits your fancy. I would go with either 3/8" ID 5/8" OD, or 1/2" ID 3/4" OD. The thicker walls help with easier bending and preventing kinking. 3/8" ID if you like the look of smaller tubing or there will be tight bends (smaller ID bends easier), or 1/2" ID if you like the look of large tubing.
Fittings: Must match your tubing dimensions, otherwise, get what you want. Barb fittings are much cheaper, but require the use of clamps. I prefer worm gear clamps, they're much more secure. Compression fittings are much more expensive, but also look a lot nicer.
Fans: Most 140mm fans perform about the same. I would get something in the 1000-1200 RPM range, as that is the most common range for 140 fans.

I would personally go for the 420. It would be a lot quieter for the same cooling capacity, and you would lose at best 2 5.25" bay slots, and you can always use those for something short like a fan controller. You would probably only lose one because that is a slim radiator, and you can put the fans on the top where the vents are.

You can get a 240 now, and if you feel like you need more, just buy a 420 and then put the 240 on the bottom of the case (the Switch 810 supports this natively).