What is the best UE4 PC setup? (Mainly CPU) And why?

gtx 1070 with broken fan is it worth
of buying and how much to spend on it
because my friend is selling?

Broken fan? £150 tops.

Also is it better to use 1070 than two
960 oc 4 gb ?

1070 is better, hands down. 960s are probable faster but SLI is simply not worth it in most cases.

Now is it ok to put OS on SSD 960 EVo
m.2 500 GB?

It’s more than OK. Pretty much a must.

I have now overclocked 3930k to 4.2
GHz as far as I can see nothing too
much beter is on market.

There’s a lot of better stuff out there (8700k | 9900k) but since your have a 3930k, you should be fine for quite a while.

for maybe 30% speed more but price is
150%…

Precisely.