Is it better to buy a relatively old bike from 0 - $400, and upgrade it's components to relatively latest ones, or buy a new bike around $1000 with it's mid level or entry lvl components..THANKS

Buy a relatively new used bike for less than $400 and only replace what you have to.

Just depends on what you want to do, I bought a $2000 bike and dropped a couple hundred on it right away. The bike didn't "need it", but I wanted a little better performance, some people understand and others think I'm throwing away money. Do what you want to do, because there really is no clear answer to your question.

I don't think there are many bikes out there that are set up perfectly from the manufacturer for any individual. Most bikes no matter the cost will probably have something the rider would like to change, for example: a bike seat, gearing, or better brakes

Much of the answer will depend on your personality. I personally enjoy tinkering with stuff and making older stuff work better than when it was new, so I enjoy buying used bikes and working with them to make them what I want. I live in an area with a good used bike market, though, and parts are plentiful on the US eBay site and even on local used advertisement sites. I also have room to work on and store more than one bike, so I have a few older bikes that I ride regularly.

If I did not enjoy working on older bikes and/or if I had room for only one bike, then I'd probably have one newer one, and probably one the costs more money.

Neither method is right or wrong -- it will just suit you better or worse depending on what interests you.

As i live in Kosovo, there is not much of chance here tu buy a used bike and find all the needed parts to fulfill it the way i want..Do you know any website where i can find used bike parts either than ebay or amazon or aliexpress?

I would say that Craigslist is the predominant website for this in North America. Kijiji is also popular in some areas. eBay can be a good option for used parts, but international shipping might make the cost prohibitive in your situation. It sounds like your area may not be conducive to buying used and also finding good parts at a reasonable price to fix it up or make it your own. You may find it better to buy new.