The paper used here is very much a matter of what you have available, what you intend to make out of the finished product, and what you can afford. I prefer machine- made watercolour or sketching paper, for it is cheap enough , but thick enough to survive the wetting. Expensive handmade papers are for most purposes, a waste of money here, unless the use that you intend to put the marbled paper to forces you to utilise this paper. The reason for this is because expensive sheets may be spoilt by an errant air- bubble or similar fault.
Another thing the marbler must take note of is the fact that some of the papers on the market are heavily buffered against acid. This is a disadvantage to the marbler, for the alum is mildly acidic in nature , and the buffered paper would neutralise the alum, and as a result, render it useless.
Because of the alum, marbled paper is, naturally, a bit acidic, but let that not worry you, because most purposes so not require such a level of acid-freeness, and besides, I posses samples of marbled paper from 1851, which are as clean and bright as ever, and the book to which they are attached shows no sign of deterioration.
Also, avoid the shiny or overly sized papers, for, they would reject the alum, and the colours.
However, I can only do so much. You must experiment to find the best sort.