Buying a slatmill
There is no meaningful budget end of this market. The things that separate mills are size, bearings and frame.
Size it to the stride
The running surface has to be long enough for the dog to move at a natural gait, not a shortened one. This is the single most common sizing error, and it is worse for long-backed and larger breeds.
Bearings and roll
How freely the bed runs decides how the mill feels and how much of the effort goes into useful work rather than fighting the machine. Poor bearings are the usual reason a cheap mill is unpleasant to use.
Frame and stability
A mill that flexes or walks across the floor under a driving dog is both unpleasant and unsafe. Weight is generally a good sign here, at the cost of portability.
Adjustability
Some mills allow incline or resistance adjustment. It is useful for progression, but it is secondary to getting size and roll right.
Building versus buying
Plans circulate widely and building one is achievable, but the bearing assembly and frame rigidity are where home builds usually fall short — and those are the two parts that matter most.