For the record I am an engineer. I've also played riggers. I am often banned from playing riggers b/c I tend to overwhelm the GMs.
I think that SR4 does need a vehicle creation system but it needs to be radically dumbed down. I don't care about the liquid metal cooling system or the molybdenum alloy frame. I want a lower Sig and more Body. (SR4 still has Signature, right?)
So I suggest something simple to make base vehicles. This is off the top of my head and for demonstration purposes:
Frame cost: (Body + Armor + CF) x Vehicle-Type-Multiplier
Engine cost: (Body + Armor + CF) x Spd x Accel x Engine-Type-Multiplier
Brakes: (Body + Armor + CF) x Vehicle-type-multiplier
Sensors cost: Rating x sensor-multiplier
handling cost: Rating x (body + Armor + CF) x handling-multiplier
Signature: Rating x Frame Cost x Engine Cost x Signature-muliplier
Armor cost: Rating x (Body + CF) Vehicle-type-multiplier
Frame cost is based on the intended weight, size and purpose
Engines need enough power to move mass and accelerate it.
Sensors don't give a crap about the rest of the vehicle
Handling is highly mass dependant
It's harder to hide a barn, and really hard to hide a 200mph barn
Armor is based on size and how easy it is to beef up.
Add a couple of simple tables and the irritating detail work of comparing the costs and looking for edge cases and you have the rules.
Upgrades should be available but no way as detailed. In SR4 having a +/-2 is a massive change so you really wouldn't need many variants. Have the cost of upgrades be based on the original ratings in keeping with the SR4 karma costs. After all, tightening up the suspension on a typical sedan has a large impact on handling and can be done fairly cheap but getting an equal boost out of a Porsche is complete bear.