Should I buy a smart TV or a regular LED HDTV and a set top box?
What set box do you recommend?

It depends on your coutry, lets assume the USA. Buy a Samsung SMART TV. Stick with a major brand (and fairly recent model e.g. released this year so that it can play HEVC H.265 videos which is the latest compression codec) and be sure to buy a 4k Ultra HD TV, so that it is future proof. If it was the UK, the answer would be a 4K TV, but it being SMART is not an issue (because of the content service providers).

I will be in the market for one soon and will probably go for the cheapest 4k SMART TV with great reviews from Samsung, JVC or LG. If it costs more than you want to pay; still buy because otherwise you will wish you had bought the 4K one with that becoming the standard now very quickly.

Thanx a lot for your answer. Still, can you recommend any articular box?

Well, to be honest, I can share my personal experience because this is the only advice that I can give. Check out this infomir mag 420w1 I`ve been using for a year already.This is a powerful and cost-effective solution for quickly launching IPTV/OTT projects. Thanks to the ARM Cortex-A53 processor on the Hi3798MV200 chipset and the built-in HEVC codec, this set-top box easily reproduces 4K content at 60 fps and does not create excessive network loading.