I want to build a commuter porteur bike and would appreciate some help selecting a frame. I don't know much about frames etc but have read that a low trail Randonneur frame would be ideal, thus me posting here instead of the commuting forum.

Here are my requirements:

* horizontal drops for an internal hub (sram automatix). How effective are the adjustment screws on the drops that frames such as the polyvalent have ? Worth it or are they a gimmick?
* able to accommodate coaster brake - needs a strong rear triangle ?
* geometry conducive to heavy front load - is low trail the only requirement here?
* Possibly front disk brake it rains a lot where I am but I know this adds weight and complexity. Might be able to get away with rear coaster brake and front cantilever
* Cantilever/disk brakes to allow for fenders
* Fender room and attachment points
* Wheels...not sure if 650B gives me better options for commuting tires
* Doesn't need to be especially light
* As cheap as possible given the above requirements - I want as little money as possible chained up on the street

It seems like a VO polyvalent is an ideal candidate except for the cost...

What exactly am I looking for here ? Could I buy a cheap hybrid or mountain bike frame 2nd hand and then get a raked fork separately to get low trail while keeping costs down ?

Any help appreciated.

Nick