Polish, Czech, Slav... similarities?

Good evening.

Purely as a hobby, i've decided that i'd like to pick up a European language (I already speak a couple Asian languages).
I initially wanted to learn Czech, but i was comparing populations that speak languages in Europe and found that Polish outnumbers it and that it also has a lot in common with slavic languages.

tl;dr:  Anyone who speaks Polish or Czech, or any combination of those Slav languages, are there similarities?  'ppreciate your comments.

West Slavic languages got too much consonants per single word and they use Latin script which even makes reading harder because of diphtongs. Adjectives have very few vowels too (svenska, polska, cesky, etc). Well, I never heard the pronunciation, but for example Polish script makes me think that this language normally has 4-5 consonants and 1-2 vowels for a word. Am I right?