Now what is the best direction to go, should I buy new one now or wait until new release?
This has been done to death in other threads - read around.
Summary: if you urgently need a new computer, the 2017 iMacs are good machines, proven tech, and include features like expandable RAM (in the 27") USB-A, Ethernet and SD card slots that might
disappear from any new design.
If you can wait, its plausible
that new iMacs will be released in October and, if so, these would probably
include the new 8th gen 6-core processors - which would be a useful speed bump for multicore-optimised jobs. There is no guarantee if/when this will happen, though. Also - on recent behaviour - don't be surprised at price rises or "improvements" that don't please everybody!
Since we're now in the middle of September, its probably worth waiting a few more weeks - the only risk being that if Apple releases a turkey or hikes prices, used/refurb 2017 iMacs might get scarce/expensive.
If you decide to get a 2017 model: avoid the 1TB hard drive/fusion drive models - at least get a 2/3TB fusion drive (which has a more generous SSD component) or, preferably, go for the largest SSD-only model your budget will bear and add external drives for "bulk" storage: the big performance boost comes from having the OS, Applications, VM, temp. files etc on SSD.
For the 5k models, get the standard 8GB RAM from Apple and add a third-party 16GB (2x8GB) kit to bring it up to 24GB (with an easy upgrade path to 32GB).
Oh, and frankly, I'd give Mojave a few months and an update or two after release before taking the plunge - despite the beta program there's always some
glitch or key piece of third-party software that needs updating when it hits the masses.