If I get a V4....
 

What are the critical option parts that are needed please?

None, car is really good out the box, never had to change anything or upgrade.

Upgrade
 

The only upgrade you will need is the tresery alloy servo saver as the plastic kit one is weak.

Sounds too good to be true lol. What diff oils then please? Used to ball diffs rather than gear....... Sorry

I tried lots of setups but settled with 10k front and 5k rear

I've had the plastic servo saver in my V3 for 2 years...

i had the plastic one in for ages and i heard people saying that they break them and i was like how? then i looked at the part of mine that is hard to see when in the car and there was a slight crack in it. so i got the durango alu one, but car is fine without upgrades, i personally like some of the bling and alu hexes but thats personal preference

What springs too? Mid west is my region normally so astro and occasionally grass. Get it all ordered together....

Kit springs work fairly well tbh. Saying that I preffer red fronts (dark blue is kit) with kit (light blue) rears.

Upgrade wise I've got a lot on mine but 3 that I would say they offer distinct advantages.
Rear diff inserts
Rear diff holder
Servo saver arm
The 410 is a strong car, and these aren't 100% nessesary but are worth it. Hth
Charles