what suspension fork should I get
It's a difficult question to answer as there are a lot of personal variables. 
"Nothing too rough in my mind" being the biggest variable.
For a lot of flat or rolling terrain without many obstacles; rock gardens, gnarly roots, etc, I would say the stock fork is fine. I have an old 26er with a similar fork which does fine on fast rolling trails but sometimes bottoms out on steep but abrupt drop ins.
However, to be honest, if I was worried about the fork being able to handle the terrain, I most likely wouldn't even consider riding a folding bike on it.