The shape of the firebox will cause lots of problems with removal of ash, creation of clinkers and in general poor performance when burning coal.. Now, if you are in bituminous country, and plan on burning Bit coal it might be just ok..
I'd highly recommend buying an indoor coal boiler, putting up a small building to enclose it, and save a ton of money doing it.. outdoor boilers in general just don't measure up.
Do a search for OWB on the site, you will find plenty of info about NOT using an outdoor wood burner for coal.
The grates shown on the website do look nice, but the shape of the firebox above will still cause the boiler to have constant problems with clearing ash and causing clinkers to form
You can also do a search for US Stove , their 'Hotblast 1557, Clayton models can burn coal, but only with lots of attention and modifications.
Hope this helps.