Minivan conversion

Lithium battery pack in a large, un-aerodynamic vehicle for "not a lot of money" is the definition of contradiction. Lithium, is expensive, plain and simple. Cheaper over the (very) long run than lead-acid, but you're looking at minimum $6k.

If you take out the seats, and you don't need any of that space, well I suppose you could haul around a lot of batteries and make it a pretty high range, decent speed EV. But a rather useless minivan. Ifyou're okay with losing the minivan functionality, well, I guess you could go for it. You can probably even keep the second row of charis and just ditch the back row and have plenty of room and weight hauling capacity. I think four motors is a bit of overkill though. My minivan has a fairly small, 3.3L engine. You can replace something like that with one beefy DC motor.

If you're looking at Seimens, that probably means you're looking at AC. AC is much more involved and usually more expensive. I wouldn't really recommend it for cutting your teeth on, unless you're already familiar with the required electrical engineering to create/mate an inverter to the motor.