Ehr....

You should REALLY not expect anyone on this board to know a game you are talking about, not matter how popular it is (Yes, I know GTA 5 pretty well, I have never played it and never will, so don't expect me to actually know what you are talking about if you go into details without explaning yourself)...

Moreso with a PRETTY obscure game like the one you were referencing.

I did you a favor and googled it. This turned up:

http://www.pcworld.com/article/2045591/review-the-gaming-is-thin-in-free-survival-horror-slenderman-the-eight-pages.html

Apart from the game aparently being not so good (and the whole Slenderman thing sound like a rather lame internet fad, but what do I know), I found the image in the middle of the page showing you the resolution and graphics option rather interesting... why? Because this is pretty much the standart Unity startup dialog, plus the Slenderman image replacing the Unity branding.

I would bet this Slenderman game has been built with Unity also, so at least part of your question has pretty much been answered.

Yes, you can do it in Unity. If it is viable as a first game is a different question though. Looking at the images of the game, it looks pretty basic. 3D, yes. Crappy textures and low poly objects though.

IF you are able to use Blender beyond absolute beginner level, you should be able to produce this quality of 3D objects pretty fast. And at that stage, you could get objects of similar quality for free from the web without any problems.

Given you don't go overboard with the unnecessary stuff (effect, viewdistance, and so on), and you don't do something stupid (using 1000 of meshes without batching or combining them), you should also not see any performance problems.

Just a quick word of warning. Never underestimate the amount of work that even goes into a basic, dated looking game. Game development is A LOT of work, even if you manage to cut corners. You should still calculate about 6-12 months for such a game, given you scope it reasonably (read, you don't go overboard with dozens of levels or dozens of hours of gameplay).

Now, go and do a better job than the devs of this particular game apparently did... hopefully yours will get better reviews

Also, unless something has changed overnight, IP laws protect indie studios as well as AAA studios, so if you're planning on releasing this publicly you may want to consider that.

AFAIK this Slenderman thing is something of an internet phenomena, I guess something like rickrolling, just less well known or funny. At least that is what my very quick googling tells me.

So as long as the TO is not ripping off the other Slenderman games completly, he is most probably fine as nobody really holds the IP to the Slenderman idea.