No matter what, you are going to pay way more to build one than buy one ready to go. WAY more. You're also going to likely pay a lot more for a garage coach than just pulling a car on a dolly or small car trailer. Plus, you won't have near the interior living space if you have a garage coach. There are a handful of dealers that specialize in Truck Conversions. Find one as close as you can to where you are and go check some out. Racingjunk is another great resource. You've got to navigate to the rv's truck conversion area.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio