first things first
your not going to want to use truck batteries, they are just not made for deep cycling
you are also not going to want to use marine deep cycle, they too are not up to the type of repetitive
cycling you will be doing.
look into true deep cycle batteries, the entry level are golf cart batteries such as the t105 trojan
and the upper end of flooded cells are the rolls/surrette batteries
there are other technologies, such as agm, and gell, each taking specific charge regime
flooded cells would be the direction i would recommend because of their rugged nature, hard to kill them
by charging at too high a voltage a time or two, some technologies are quite unforegiving and can be damaged
if charged improperly.
the bosch unit is a nice one, but will only reach the 70% efficiency at ~80amps, and go way down as the amperage
goes up,, of course they don't tell you that.
likely you will need to do some testing, but my bet is the engine is going to return its best economy at near peak load
or around 4hp, so you are going to want to work it there, that works out to about 3kwatt mechanical
i would target for 2kwatts electrical as a max to start with. thats 66.66%efficiency alternator and drive system.
now the trick is picking the alternator, matching the drive, and putting it all together.
your going to want to study up on batteries, charge controllers like balmar, xantrex etc, voltage's and inverters a bit
to start with so you can get the best result for your money.
ymmv
bob g