I built a diy trolley,... then rebuilt it smaller and lighter,... then decided a van mount was the way to go, so I re-used the parts making a DIY vanmount.
Matt's site is great,.. all the info you need starting out.
On a practical note,.. I never got on with a trolley. Too much time spent filling barrels, to much back ache from lifting barrels in and out of the van,.. and lifting the trolley in and out for that matter. I'd recommend a cheap backpack (£61 delivered off ebay!) and a lightweight foldable sack truck to start with. Its cheaper, easier, and it'll still come in handy even when you've got a van mount for those awkward access situations.