I put together something similar to what you are doing a year ago.  We found a 9 year old 48 foot drop deck trailer for $13,000.  We put a 4250 gallon tank on it for $4,000.  It has baffels in it.  We put the tank on the lower portion of the trailer, which I felt made it feel and look safer.   We had a guy put a dump valve on the rear axle of the spread axle so now it turns corners sharp.

When I was looking I talked to the folks at www.jetcompany.com and they would build me a new trailer any configuration I wanted, and the price they quoted me was in the low $30,000 range.