I used to ride one. Rode it for a couple years, sold it to a family member 2-3 years ago and he's still riding it.
I really liked it, but I was ready for something new at the time. I thought the back end was very soft, if that makes sense. It was definitely a more comfortable ride than my Winspace.
I did have some headset play for a short time with the Velobuild one piece bar, but eventually got it sorted. Switched to "The One Pro" one piece bar shortly before selling it and had zero issues with this setup (afaik it's been free of any issues since). This bar can now be found on Aliexpress for cheap under a handful of different brands/names. If you go through with the purchase, I would definitely recommend ordering your frame without a handlebar and sourcing a better aftermarket option.
Also note that the largest tire I was able to fit in front was a 30mm Panaracer Agilest, but ymmv depending on your wheel setup. I eventually settled on 28mm Panaracers and 29mm Enves. I believe the current owner is using 30mm Panaracers (might be 28, not entirely sure).

I think it's a good purchase for the price. I might be buying back the frame soon as the current owner is looking to sell and build something new.

Spray Max 2k will work great.
I've done a couple paint jobs with raw carbon sections, and I hit it with a coat of adhesion promoter first. Upol or Spray Max works for me. Note that Spray Max has a very slight silver pigmentation to it (looks almost like a very fine silver flake, I kinda like it).
In my experience with raw carbon, you have to look out for tiny pinholes or uneven surfaces and if you find any, fill them before applying any kind of coating.

Also, I have a SRAM Force crank on my bike that I stripped and then sprayed with 1k satin clear coat and nothing else. No issues in the couple years I've been running it.

A couple more options:
- Spray.Bike has a "clear carbon primer" but I've never used it.
- Fantom Clear CX. Claims to be a clear coat designed specifically for automotive carbon fiber parts. I'm very curious but never tried as their minimum order amount is quite high.

My suggestion (if you're building a Chiner) is to just order the frame in gloss carbon.

I had these. Not very suitable for integrated bars as the hose is very stiff. Normally I use Trickstuff Beta Kevlar hoses.

This is true, it's stiffer than the original hose. In my experience it's still possible to route in a fully internal setup but may not be the easiest solution for those who don't do this often.

Here's a good budget alloy option for those of us in USA: https://www.leaderbikesusa.com/products/801

Double the price of a Gas Cycle frame, but still "budget" when compared to mainstream alloy offerings. It's even UCI approved 

This has worked well for me: https://www.omnicalculator.com/sports/chain-length

Nice!

The seller wasn't sure if it came with a mount... So I bought one on Aliexpress.


And about SRAM caliper adapter? Do you know anything?

The adapter should come with the shifter/brake set. If you don't have one, you can find it on AliExpress for a couple dollars/euros/etc. I swapped mine for this: https://www.aliexpress.us/item/3256808983914499.html

I’m interested in buying! Would be for shipping (east coast US). What version of the ER9 is this?

Hello! Yes I can ship. I believe it's the first version.

You can also add a normal round 3 or 5mm spacer on top of the handlebar and use a standard top cap to achieve proper compression until you can get the fork cut.