Suggestions for budget mountain bike with opportunities for upgrade I live in Bulgaria and I am looking for suggestions for budget mountain bike with opportunities for upgrade, budget 400-500$ . I am planning to travel from home to work (dayly) and to take part in competitions like 100 km (about 63 miles) mountain lap called the "The 100km Vitosha Lap" and to have occasional drifts in the mountain .

My choice so for goes for Drag Ontario Pro. I have heard good things about Sprint Apolon too, but I am not so impressed. 2nd hand RAM or Scott are also an options.

I am interested to hear suggestions and if you don't know what to recommend me in that price range advices what I should be looking for when buying a bike with options for later upgrade.
Assuming a new bike, look at the Scott Aspect 30, 20, or 10, depending on your budget.
They all use the same frame, and the only difference is components.
When you buy a bike with the intent to upgrade, realize you will spend far more long term, than if you bought your bike ready built with the same components you upgraded to. I understand why it can be better to do it that way to break up the cost, but it does cost a lot more money overall.
I would make sure you buy a bike with good wheels, a good frame, and a good fork. Those parts are astronomically more expensive to upgrade to than to buy on the bike.
Your budget is fairly low, so if you have the option, save a bit more before you buy. A budget of $800-1000 is more appropriate if you can afford to make it work. It allows the wheels and fork to be quality enough that upgrading the other components becomes worthwhile.
If you can make that budget work, then look at the Scott Scale line.
I like and recommend Scott bikes for you because they are readily available in Europe, and they make high quality bikes at a reasonable price.
I hope this is helpful to you.