Arms reach - help please!

[Londontwinmum]

Hi
I really need some advice on the arms reach-I just bought a second hand original co-sleeper for my twins who are due any day now.

I know we need to buy a new mattress but should I just buy a standard foam one to go on top of the hard one that came with the cot or can I not use the hard one as its second hand?
Otherwise I'll need to construct some sort of base to go on the poles?

Also - does it matter if the mattress and sheets aren't arms reach branded as they won't have the straps/Velcro??

Also- what size!? I know it needs to be a good fit but because the sides are flexible you can still get your arms down the sides unlike a wooden cot - is this a problem?

I'm a really nervous FTM so would love some advice. I don't want to get it wrong due to the safety concerns.
Help!

Thank you