I've got tons of Lego - mostly Technic. I build Lego trucks for trialling - there are some basic rules - approximate scale of 1/14, must have a truck load bed, use the Lego engine with pistons, only allowed 1 drive motor, as many other functions as you like (locking diffs, air suspension, automatic gearboxes etc.) and as many axles / wheels or whatever suspension designs you like.
It's surprisingly difficult to transmit huge torque without shearing parts - but that's all part of the design challenges.
My wife is more into the building sets - she's got the Lego trainsets but now she wants the Disney Castle - but that's about £350 so it'll have to wait.