The 2007 3-valve V10 does not need EGR delete like earlier versions. Focus on regular maintenance: Motorcraft spark plugs, synthetic oil, and transmission fluid changes. Engine is reliable as-is.
E-350 and E-450 use same V10 engine with nearly identical 8-10 MPG fuel economy. E-450 has heavier suspension, brakes, and higher GVWR. Choose based on weight capacity needs, not fuel economy.
Yes, there are aftermarket transmission controllers available for your 2007 E450 that can hold it in Drive gear. The most practical solutions include installing a manual shift valve body kit or an electronic transmission controller.
SDI E-Clik owners report noticeable body roll reduction and improved handling. Expectations should be realistic - its good for the price but not a full suspension replacement.
Banks, UPFORD, and 5 Star V10 tunes all deliver similar improvements for RV use. Since you are happy with UPFORD, transfer your existing tune to the new coach. Transfer fee is more cost-effective than buying new tune from different provider.