I don’t think there’s anything that meets all of your requirements. In all cases I can think of, you’d have to do at least a little work. Your final requirement (step-by-step instructions, without headaches) may be a little pie in the sky. I don’t think anybody offers anything like that for UE4. Remember, UE4’s affordable, subscription-based version has been out for less than a year, and has really only been getting attention of the indie game community for the last 5-6 months.
The problem with stock characters is that they tend to look like stock characters, and Unreal users, historically, have been higher-end game makers that would have their own in-house artists working in a game-specific style.
The options I can think of:
Mixamo Fuse: Coming along nicely and I’m excited what 1.4 will bring, but currently, it’s not usable for our needs except for prototyping. There’s no LODs and there have been some issues with UE4 import (issues Mixamo says they’re investigating). Fuse models can auto-rigged to the UE4 skeleton and then downloaded with morphs that can be used with their facial capture software. I imagine those same morphs could be hand animated, but I’m not sure how hard that would be. This is the easiest option for getting animatable characters, though, assuming you have a subscription to Mixamo, since the auto-rigger will now rig to the UE4 skeleton and use the stock Unreal animations. Mixamo claims their decimator tool works for doing LOD models. I’m guessing it works fine for LOD3 and higher, but I’m a little skeptical about LOD1 (where LOD0 = full resolution Fuse model). I plan to try it since I’m a Mixamo subscriber now that they have full UE4 support, but haven’t gotten around to it. ://mixamo/fuse
MakeHuman: An open source project with a lot of potential, I tend to find the models need some modeling and texture work after export to avoid “looking like Makehuman models”. They offer several rigs and topologies and, I believe, facial animations are in the development branch (but not in the last official release). I’m not sure how close to production-ready they are. The models should be usable with UE4 animations using retargeting and choosing the game rig, but I haven’t tried it. There’s no explicit LOD support, but there are multiple topologies with different mesh counts, so you could probably get at least two LOD levels out of it, and then decimate down to third. ://makehuman.org
DAZ Studio: The DAZ models have really great topology and are rigged well, but they’re very high poly. They also have to be licensed for game use, and if you use third party textures, clothes, or morphs, you also have to license them separately from each vendor, so from a legal standpoint, you probably want to stay away from it. The ability to quickly prototype a character of different age/build/race/etc. is nice, though, and I will often use DAZ models for in-house prototypes. The legal situation makes using them (even retopo’d versions) too risky, however. ://daz3d
Unreal Marketplace: There are a few character packs. None of them currently have casual clothes (except, perhaps, you could argue that the biker in the Hero Character Pack is very casual 
). I’d keep an eye on it, though. As the number of UE4 subscribers increases, I expect to see more offerings here targeted at indie developers. You can check the Trello page to see what’s in the pipeline and vote on what you think should be put in the marketplace. https://trello/b/x2AEJP0x/unreal-engine-marketplace-submissions
TurboSquid: There are many character models there, some of which are game-ready and may meet your needs. There’s also a lot of **** there, too, so you have to spend time looking for something that meets your needs. ://
Unity Asset Store: I’m hesitant to suggest this here, but the Unity Asset Store has much more extensive offerings than the Unreal Marketplace currently. It’s also not as well curated, so there’s a lot of **** over there. Also, most of the assets come as Unity assets only, but some of them come with FBX files, and most of them have contact information where you can reach the artist if you want to ask them to submit to the UE4 Market.
Partner or Hire: In all honesty, your best option is to partner with artists or hire some. There are many people looking to collaborate on games, and modern 3D games are really tough for a single person to do alone. There are also a lot of artists who post LFW (looking for work) posts in the forums and many of them have excellent portfolios and reasonable rates.
I’m sure there are other options that other people will chime in with but, frankly, most stock options for humans look like stock options, so they’re not an option that we use except for strictly in-house prototyping.
P.S. Whoah, the forum’s auto-censoring makes me look pretty foul-mouth. The censored word was carp, but with the a and r reversed, not something most people would consider a profanity. 
