The non-K series are what Intel calls "Partially Unlocked" since Sandy Bridge. You get 4 bins (1 bin is 100 Mhz, or whatever the base BCLK is set to, in this case) on top of the max turbo clock. So the max you will be able to do with the 4771 is 3.9 Ghz (max turbo) + 400 Mhz (partially unlocked multipliers) = 4.3 Ghz max. With a small bit of BCLK adjustment, you can get 4.4 Ghz (most likely stable).
If you are bent on OC'ing more than 4.3 - 4.4 Ghz, you have no choice but to get a K-model (and a good cooler). Otherwise, you can get away with a 4771 just fine. However, the price difference is nearly nil between the two models, and they both Turbo to 3.9 Ghz, so there really isn't much of a difference. If your priorities with a computer is as you listed, you won't miss any of the pro features on the non-K models (that's right, the K models lack certain features like Trusted Execution). If the price is nearly identical (within 20 bucks in my opinion), go for a K model would be my suggestion.
Most motherboards offer some form of "Turbo Enhancement" where all the cores are working at the max Turbo speed, not just 1 or 2. So you can usually count on the 4.3 Ghz as your all core max speed.
As for system limits, usually you get plenty of warning signs before something goes seriously wrong, such as crashes, and unusually high temperatures (on the CPU or the VRM on the motherboard). You can always establish a good baseline by reading a review or two on your preferred motherboard, to see what the reviewers have said about the OC capability of that board. Most of these boards also come with a tuning utility (in the BIOS/UEFI or in windows) that allows you to essentially push a button/click once to automatically overclock, which is great for beginners. That option is very safe and usually takes you to with in 15% of the max your CPU+Board+Heatsink/Fan solution is able to do, and that's typically enough, unless you plan on beating records.