Will there be an iPhone SE 2 at Apple's March event? When will the March event be ?

Apple hasn't confirmed anything for March Keynote as of yet. Also, if there is an iPhone SE update, it will likely only be in storage. A completely revamped version would not be likely until 2018, if we see a revamped version at all of the iPhone SE.

 

Isn't the March keynote always at WWDC?

 

No. The March keynote and WWDC are two separate events. The March keynote is primarily hardware-based and is obviously in the month of March. WWDC is regarding developers and software. WWDC also hosts iOS, watchOS and macOS demos. WWDC is held starting June 5th.

 

I don't think there will be a new SE2 this year; but they may update the storage to 32GB and 128GB similar to what they did with the old iPhones and iPads when the 7 came out.

Aside from the usual people like Ming-Chi Kuo saying no new SE2 is coming out we don't have any leaks of a new SE by today.

Looking at what happened last year, we had rumours of a 4" iPhone coming out for weeks, then leaks of the name "SE" and actual pictures in the fortnight before 10 March when Apple announced a special event on 21 March.
Looking at 2017, we only have rumours of a 128GB version and suggestions of s 32GB version.