Sleeping in Pack n Play

We have planned for our baby to sleep on the bassinet level of her Graco Pack N Play (On the Go Playard) in our room until she’s ready to go in the crib.

I know she’ll need a sheet on the mat, but now I’m seeing some people buy a mattress for it? The mattresses look like they’re another $50 so I don’t think we’re doing that.

My questions are (1) what sheets to buy, and how many and (2) Do the sheets get tight enough since there’s nothing below to tuck them into, (3) is the mattress a better idea and if so, should I just get a bassinet at this point??

Thank you!