Hello Necks!
A friend of mine was suggested to use an A***F, 5/8 Wave 2KW vertical FM Antenna for his 600W FM transmitter.
I would appreciate your opinions about that and your suggestions on a cost benefit antenna suitable for 1KW, to be on the safe side.

Really don't consider any A***F gear - it doesn't work.

What kind of coverage does your friend want to get? Does he want to radiate equally in all directions, or can he shape the coverage to aim for centres of population?

The projected coverage area determines the kind of antenna that will work best: I used a very high site to the South of London, and wanted to get a signal into most of London. There was no point in broadcasting southwards - to sheep and trees - so I decided to use an aerial with a hemispherical radiation. I used an "H" to start with - a 2-element Yagi - which steered the signal northwards, and provided some worthwhile antenna gain: more power towards the coverage area at little expense! My next antenna - which gave even more signal for the same transmitter power - used stacked "H" aerials. Once the phasing harness was tweaked, the signal was colossal. There was some "spill" southwards, but >95% of the signal was heading towards the area I wanted to cover!

Really do your research. Consider the use of "stacked" aerials and directional aerials to enhance your signal....

Colinear dominator is good with high gain and onmidirectional, high power too