Depends on how do you define serious. Valve seals replacement can take you back around RM1,600 and tensioners can be replaced around RM800. Means that money that is suppose to be spent on mods, are instead spent on keeping the car in good condition. Those are issues commonly found in 320i engines, but it does not mean that all 320i engines are like that.
Same as 335i and 325i engines, each has their own common issues. I was driving a 325i engine for 4.5 years, and the only breakdown I got was due to water pump, in which I replaced. Not a serious issue, but it's money that I need to spend not on mods. I also had the O2 sensors replaced when it got burned out, also money I did spend just to keep it running. In comparison to the 320i and 335i engines, I think the 325i has less engine issues. The only issue I have is that the 325i is slow in comparison to the 335i.
So I recommend, if budget permits of course, to get the 335i. Preferably the one that is for sale by a Club member.