The cheapest kit that I found for the E350 / E450 chassis was Big Foot Platium system. I love mine (Axis on E450 Chassis)..... works great.
But there are several brands on the market. Pick the one you like best.
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, TPMS, Surge tank, Tornado flush, Viair Compressor