I started in the bed of a pickup. Got a trailer and would NEVER go back. Loading and unloading is a pain and not possible when you start getting more and more equipment. Also if you do shrubs...where are you going to put the clippings? A trailer will allow you to grow more and take on more work on a daily basis.
A 36" Commercial walk behind weights about 500-600 pounds. That's a lot of weight to control coming off the ramps. It's asking for trouble... "IF" you insist of working out of the bed get the Timemaster 30" mower. Much less weight and a lot easier to take on and off the truck. Probably the best start up mower in your situation.
I agree with the above post, get a 5 x 8 trailer THEN get a 36" walk behind.