Unipack / Create / Sports Direct singlespeed track bikes?

You didn't post an opinion: it was a criticism of the advice previously offered (to buy 2nd hand).

Chain's and cassettes are consumable items, so whether you buy a 2nd hand bike or a new one, they will need replacement.

Why you think a used bike will probably need a new crank is anyone's guess.

Perhaps the advice to buy 2nd hand needs clarification: "don't buy a fucked 2nd hand bike".

I think the advice to buy S/H is fine but somebody with limited knowledge should be aware that there could be untold problems with a S/H bike. It's not the answer for anybody who doesn't know what problems to look for.