What should I look for in a new front wheel for my heavy body?
The things that will make a wheel more durable in this kind of service are:


Bigger tires – the bigger the tire the more space you have to cushion an impact, bigger tires also mean that the load is more distributed. Since the bigger tire gives you more support you can also run a somewhat lower pressure which means that there is more flex in the tire before the rim takes the impact.
The right tire pressure (especially since you may not want to change your tires). Too much pressure will make the tires hard and will transfer more of the impact to your wheels and the rest of the bike. Too little pressure means that you'll be getting pinch flats and impacts on the rims – also not good.
Proper spoke tension – the spokes support the wheel, making sure that they are tight enough and evenly tensioned will ensure that the wheel is getting as much support as possible.
Deeper rims – a deeper rim section will be stronger under impact. Wider will help with lateral loads. Going too wide will straighten the sidewalls of your tires and may actually reduce their effectiveness at cushioning impacts.
More spokes – hubs and rims for tandems can be made with 40 or even 48 spokes, if you're already running 36 spokes and they were properly tensioned I'd be inclined to go for more spokes when you build the next wheel.


I think the width of the rim is more about matching the rim to the tire (and lateral strength) than about radial impact strength. That said, 19 mm seems kind of narrow for a 50+ mm tire (at least based on this table of rim widths from Schwalbe). If you've got clearance on the bike for a wider tire, I'd go for at least a 23 or 25 mm rim just to have options.

Another source of good advice would be a wheel builder who specializes in durable wheels. In the US Peter White is a possibility.