Engine will charge the battery via the same lead that starts it so no worties there
If your using it as mainly a start battery you can just use a fairly cheap small car battery. Something like this
Pure Leisure batteries are not designed for starting loads. They are for light loads over long periods & starting will reduce there life however you can get dual purpose kinds of halfway house batteries now.
For your purpose you can have two perfectly adequate batteries for the price of 1 leisure battery
Whilst it may be argued the leisure battery will last longer i doubt it will last twice as long