The only thing i wouldnt buy second hand is a car seat. If you re happy with the quality snd sturdiness of the second hand buggy go for it. Who cares whether it is designer, first or second hand? If it works for you that the only thing that matters.
I second your sisters tho, my second child couldnt stand the state if the art icandy peach blossom we got when she was born and that costed an arm

but loooved the mclaren xt we already have for travels and nursery. But I personnally loved the bugaboo cameleon i had for my first child (no double at the time). So see how you feel but it is also ok to indulge a bit when it is your first i think.
Good luck and yes there are so many things you dont need, be careful. X