Tower style are the type of cooler. The 'top' of the tower is what faces the side of the case since the motherboard is on its' side. Essentially a single tower with a fan (sometimes 2) like the cryorig h5, h7, cm hyper 212 evo, tx4 etc - they have 1 tower consisting of heatpipes surrounded by cooling fins. Coolers like the dark rock 3, pro 3, nh-d14, nh-d15, cryorig r1 universal etc are 'dual tower' because if you look at them, they have a fan, a row of cooling fins, a fan in the middle, another row of cooling fins and heatpipes (and sometimes a 3rd fan).
Unlike low profile 'flower' shaped coolers like many of zalman's, the cryorig c1 etc. which are more suited toward thinner cases, mitx, itx and so forth.
Overclocking value is more or less up to the user. Many 4690k's reach 4.6-4.7ghz. While it's possible to go for a more mild overclock like 4.2, keep in mind the turbo setting out of the box is 3.9ghz. Not sure why 4.2 to 4.6 ghz would be little improvement (400mhz increase) compared to 4.2ghz (300mhz over stock turbo). 400mhz doesn't sound like a lot and in comparison to frequencies of 3.5-4.0 it isn't relatively huge. Although if your cpu is busy working, 400mhz across 4 cores is an additional 1.2ghz combined processing power.
Any of those coolers you're looking at should be capable of reaching the cpu's physical oc potential. Unless you have really high ambient temps, they usually cap out at voltage before temperature. The 212 evo for it's cost and size is a good performer, just not quite enough to handle load temps much beyond 4.2 or 4.3 in most cases.
It's always a roll of the dice with the cpu binning, for my chip the sweet spot is 4.6ghz. I can reach 4.7 and stay within thermals but the voltage needs to be raised to a point I'm not comfortable with for full time use. Some (very few) people have trouble getting to 4.5ghz, I've seen others reach 4.7, 4.8 ghz at the top end. Just don't be disappointed if you don't get that high, 4.8 doesn't really seem to be the norm either.