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.