I recently did a hydraulic surge to electric conversion on a dual-axle box trailer, on a Monday in West Virginia, under pressure during a move (because the surge system failed going down a mountain).
It's not that bad, IF the hub mounts are a conventional 4-bolt with a standard pattern.
You WILL have to get new drums with the machined surface for the magnet to ride on, and you'll need to do something with the tongue to stop it from moving (I just drilled through the slide portion and put a nice stout 3/8" grade 8 bolt with a lock washer and a nylock nut).
Mind, I did all this sitting in an Autozone parking lot, under duress. Took me all day, partly to round up parts (2 trips to tractor Supply for the electric brake kit which includes everything BUT the drums, and one trip through the country to locate drums), and partly because I put the backing plates on the wrong sides initially, so had to undo and re-do those 8 freakin' PITA bolts (not really that bad, just tight space behind the plates.
Could probably knock this out in 3.5 hours if I had the parts on hand. It's really more tedious than difficult, and that I'm a bit picky about running wire nicely.
My total cost was about $300. I could probably do better shopping for the parts.
Here ya go - everything you need for $250 from Northern Tools:
http://www.northerntool.com/shop/tools/product_200514141_200514141