Walkers are fun, and I've built 3 or 4 so far. However, they're also a lot more
problem getting to work than wheeled robots. Quadrupeds are also more difficult
to get the gaits right than hexapods.
It might be easier to get into robotics initially with a wheeled bot, as there are
many issues that need addressing: mechanical, electrical, drive, sensors, controllers,
on and on. Then add on legs and gaits to the learning curve for a walker.
As a possibility for cheap and easy, you might build a wheeled bot that uses
continuous rotation servos, then you don't need an extra motor shield, most of
which co-opt "many" Arduino I/O lines [usually 6 lines minimum].
I have one wheeled bot with continuous rotation servos that I use mainly as a
development platform for my other robots. Testing sensors, developing control
routines, etc. Here is one company sells cheap platforms: