While a current Mangrove keeper such as Michelle can provide you the most information on these guys, I thought I would add some information from my past experiences with these guys.
I once had a large adult mangrove that was extremely docile. I have held ones owned by others that were also very nice, and seemed to have the same disposition, but remember that each one might be different. I owned mine for a long time and experienced no incidents, and handled him frequently. He never became disturbed when I did frequent cage maintenance, never tried to run from me, and did not find humans upsetting as well. I would compare his disposition to a Rudicollis.
My recommendation would be to get one as a baby and go from there. They are really great varanids.