If you find the standard Lego models a bit too easy, there are some bespoke options out there that are more challenging. Just sourcing the parts is a task in itself.
A great example is the Land Rover Defender 110 designed by Sheepo:
https://www.sheepo.es/2012/05/land-rover-defender-110.html
This monster uses 3,442 parts, has seven motors, five forward gears + reverse and a high and low range transfer case, working disc brakes and remote control.
Sheepo charges a very reasonable €20 for the build instructions which are of the same design and quality as those you receive with any standard Lego model. The challenge is to find those 3,442 parts. To the rescue comes BrickLink:
BrickLink is a marketplace for Lego and allows the upload of Wanted Lists, for instance the parts list for the Land Rover, which can then be sourced from the thousands of vendors on BrickLink. Be prepared though – identifying the optimum combination of orders that minimises the overall cost after taking into account shipping is a bit of a logistical nightmare.
Parts lists and instructions for the Land Rover can be found at Rebrickable:
https://rebrickable.com/mocs/MOC-30043/Sheepo/land-rover-defender-110/#details