Both engines would be stationary.
My farm is not huge but signifigantly larger than hobby size, unless you consider 35 cows and 4 full time employees a hobby.
I am looking at biogas for two reasons. One is I produce several thousand pounds of manure a day and I would like to process it rather than send it raw to the field.
Second and more importantly, my farm requires power to run and a serious disruption of the diesel supply would be very difficult for us. Cost is less important in this case then having a backup.
Your other suggestions are interesting and useful for people running smaller scale hobby gardens