When it stalls is it like a slow quiet stall or does it sound like it might be missing when is stalls? Could be PCV needing replaced, only a couple dollars for a new one, or it could be a spark related issue... The first thing I do when I get a car that is used or has higher miles on it is the basic maintenance on it. Air filter, oil and filter, check all fluids, sometimes new plugs and wires if they look like they need it and sometimes the PCV.
As far as intakes go, there shouldn't be anything special about the high dollar ones. I would just use a pretty basic cold air intake pipe matched with a nicer filter like a K&N cone filter. I'm not sure how the 420a motor meters air though. If it uses a MAF sensor by the filter or if it uses a MAP sensor in the intake. If it is a MAP sensor then you can use any of those intake pipes. If it uses a MAF sensor near the filter then make sure you get a pipe that will utilize the factory sensor or has an adapter ring for the MAF housing.
Hope this helps ya out!