Will there be certain limits with the length of your routes (ie: if you're a level 1, will you only be able to have a route of 1500km, etc)?
I'm not quite sure what you mean? Maximum lenght of route depends only on the range of planes you have.
Do real world airlines have such limits .. no (perhaps some countries excluded where government keeps tight control on airlines and route rights). ... So it's not included here, and there are no game stylish "levels" either.
so if i decide to lease a 747 right at the start, i can fly within it's limits?? i like. In aviosim, you start with level 1 and your routes can only be within 1500km (which sucks because i could buy used airbus and boeings that could fly 5000km).
Yes you can, if you just have enough money! It's not uncommon to start with for example a leased 767.