A bike smaller than is recommended I have 180cm and the recommended size for me is 58cm or so, which i already have, a classic Peugeot racing bike. Works relatively ok but i found that i ride it better when i'm leaning forward and keep my hands on the lower side of the handlebars. And now comes the problem, i'm thin and i need more power than i have to ride this bike fast and long. However, the only bike i found to buy in my country and also to like it, is a 54cm with the reynolds 531 frame but i can't test it as it's not in my city (i do plan to test other bikes of 54 if i can find but for now i didn't).

So my question is, is there any reason i should not buy it? From the way i see things it shouldn't be any problem if i can pull the sit high enough. The handlebars are already a little lower which should be good. If i look to professional riders they also seem to have a smaller bike comparing to their body size, but this Peugeot is like controlling me.

Thanks.
To me "58cm or so" means, "58cm, maybe 56 or 60." It doesn't mean 54cm. If a professional has said you should be on a 58, you should heed that. 4cm in the wrong direction could place your knees too far forward of the pedal spindles and be less efficient or even cause repetitive stress injuries to your knees.

Instead, try fine tuning your current bike which sounds like is "almost right". You have several options to make the 58cm bike shorter: moving the seat forward on the seatpost, replacing the seatpost with one that has less or no setback, and/or swapping out the stem with one that has a shorter reach. If it's an old Peugeot with a threaded stem, you can also adjust the handlebar height.

The combined cost of making all of those changes to the 58cm would likely be less than buying a different bike.