How many stones work best for you depends on your knife set and what edges you want. There's no universal best sharpening kit. Even as basic a difference as "oil or water stones?" depends on what kinds of alloys you're sharpening.
I'm not suggesting you do what I do, but by way of an example I prefer four stone sets with the idea that the coarsest stone will be very coarse and used for occasional profiling and repair; and the finest stone will be very fine and used for polishing those knives which can use it. The other two medium grit stones are for "raising a burr" (aka "pulling a wire") which is the actual sharpening process of creating a true, fine, fresh metal edge. However, if you're not looking for a great deal of polish you can certainly make the medium/fine do double duty.
I have a complete oil stone set and a complete water stone set -- some knives sharpen equally well on the different sets. With stainless at least, the difference seems to lie somewhere in the difference between toughness and strength. That is, stronger, harder knives do better on water stones.
You're going to have to make some decision about what level of polish you're willing to sharpen and maintain -- and that will determine your final stone.
On Choseras specifically: They're great stones, I have no complaint about their performance whatsoever, but they aren't worth the price. There are other stones which perform just as well and are also considerably less expensive.
Don't buy bench stones less than 8" long. For kitchen knives anyway, small stones are a false economy which make sharpening a lot more difficult than it should be. The savings are a classic false economy
If you're not looking for an ultra bargain or beginner set, I recommend the following kits:
1 and 2. Norton India coarse and fine or the Norton India coarse and fine combi stone (IB-8, or IC-8). Indias are almost as fast as Crystolons with much less tendency to scratch. They're also more chip and crack resistant.
3. Hall's Soft Arkansas. Hall's not Norton or Dan's. The quarry makes a difference. We can get into this in another post if you like, but I've already written a lot about it here and there.
4. Hall's Surgical Black Arkansas
1. Beston 500. All coarse stones suck. This is the least sucky of all.
2. Bester 1200. Fantastic stone. Unlike the Chosera 1000, it has a few idiosyncracies -- for instance it requires a long soak and it's quite hard -- but it's a better sharpener and lends itself to a big jump to the next grit level. And it's not much more than half the price of the Chosera.
3. Probably an Arashiyama (aka Takenoko). It's actually a 6K, but some sellers rate it at 7K or 8K because the polish is so bright and smooth. I like this stone as a final stone for most kitchen knives a lot. A helluva lot, actually. It's a very fast stone, very easy to learn and use, very easy to maintain, and very reasonably priced. If you're looking for more polish for some of your knives (say going to the 8K, 10K or some even higher level) you're probably looking for less polish for some of the others; and that would mean a complete rethink. But unless that's the case, it's not worth discussing.
Flattening is a big issue with water stones. I suggest you pick up some fairly coarse drywall screen, put it in a sheet pan, soaking your stones, wetting the screen thoroughly, and start flattening (and chamfering) by moving the stones across the screen. You'll need to rinse the screen clean frequently to keep it from clogging. This is a somewhat messy and slow process but it's very effective, easy to learn and cheap as can be. A life time supply of drywall screen will run you less than $15. After you've been sharpening for a few years, take stock of the inconvenience and consider investing the $80+ in a DMT XXC.
A few final words... Stones can be fairly trendy and my recommendations are definitely in the current mainstream. That said, the oil stones are my current kit (it appears I'm one of the trend makers) I've been using for some years. As it happens I got on the Beston/Bester bandwagon at the same time everyone else did. I don't own a Takenoko, but would if I wanted a 6K edge.
Also, we "experts" certainly have our favorites and stones we don't like and the differences are often irrational. There are a lot of right ways to sharpen and a lot of great stones. The best way to take my advice is to remember that it's a post on a board and not chiseled on stone tablets.
Hope this helps,