Looking at buying my first Landrover. LR3 specifically but having difficulty finding a good answer to the differences between trim levels.

I know some of the things like engine size, 7 passenger, upgraded sound system were HSE standard. But I see V8 7 passenger SE's out there. I just want to know what else I'd be missing out on if I got one of those.

 

There are two US engines, the V6 and the V8. If the vehicle has a V6, run! You could find the other trim options on either vehicle. It might be beneficial to decide what options you would like, and match things up that way. My SE has the 7seat, NAV, locking rear diff, rear A/C. Notably missing (and missed) heated seats, seat memory, reversing mirrors.

So, it can go either way on trim levels, depending on how it was ordered by the dealer or original buyer.