Look to buy a new trailer....

Oil pan depth should not be an issue with any trailer. My last pan was 3" off the ground and I never had an issue going up on the ramps to my flat bed steel trailer. The aluminum trailers are nice - but quite expensive compared to steel (2X). Obviously they don't rust, look good and they are significantly lighter. I bought a new 18' flat bed steel trailer with dove tail and ramps a couple years ago for around $2600 OTD. The same trailer in aluminum was close to $6K. I just couldn't justify the extra cost