800 hrs, is a fail, I would be asking why such a short life span, and there are reasons that I will get to, but first I have to tell you that I too can produce hot water but I would never do it electricaly as I get more hot water from the cooling of the Diesel. Even a little 5-7 hp water cooled Diesel will produce hot water faster than it can be used. One of my gen sets has the engine making 4 hp, and the alternator is loaded to 1500 watts, I have steam coming off the 30 gallon cooling tank in about 30 minutes.
Also using a over sized head as likey you are going to do, means greater losses powering the field windings. Dont get me wrong, it will absolutly work for you, but my concern is getting the most power out per litre of fuel burnt, a big concern with fuel costs greater than 4 dollars a gallon, maybe not such a concern where you are.
Regarding brushlife, 3 thousand hours would be the minimum I would expect to get on any slip ring/ brush type alternator running at 1800rpm, which is what the ST heads run at.
I would rotate or have someone rotate the shaft while I looked at the brushes, you want to be sure they are running centered on the slip rings, if not adjust the holders (I have never seen a ST with the brushes centered right, then you want to check for brush bounce as this can really shorten brush life. With someone spinning the shaft put one finger on a brush and watch another brush as the shaft is spun. If you can feel and see the brush moving in the holder, you have a problem. Ideally they should be rock steady-no movement. You can have brush bounce for a number of reasons, worn bearings, or crap new ones, a bent rotor shaft,I have found the usual cause on these type unit when new is just that the slip rings have not been machined true, or machined down enough(easy to fix). Just sometings to check if you only get 800 hours.