Why Manual?? The Auto is really good in these vehicles.
As for Mods, I guess it depends what you want to do with your 150. Do you want function or fashion?
I put in some rear drawers, an under bonnet compressor, duel battery system with DC/DC charger and a fridge slider (+ fridge, of course).
In my new one, (2019) I also got the commercial bull bar put on, just for a better approach angle & front ground clearance. It was an issue on Fraser Island, with the standard front end, in the 2014 model I had at the time. I also put on some aftermarket, black wheels with AT rubber..... just to make it look a bit different and make it more suitable off road. The Grandtreks are not great, off the highway.
Most of the other stuff that people weigh the vehicles down with (and generally never use), didn't appeal to me or seem at all necessary.