At present, the WC51 is just fine. It comes with the withdraw and refit commander ability and can maraud around the battlefield virtually unopposed against the OKW for the first four or five minutes (until either a panzerschreck or rakettenwerfer appears). With the coming balance changes, and the introduction of snares to the Oberkommando's mainline infantry, the WC51 will become virtually useless, regardless of price and regardless of the opponent's faction. In order to remedy this, I propose that it be replaced by a raid ability that costs 35 munitions, is unlocked at 1 CP, and allows light vehicles (but not tanks) to capture territory at 1.15x speed for 1 minute. This removes a soon-to-be useless and unbalanceable ability and replaces it with something that is useful and thematically appropriate.
As i understand faust ability comes up only after truck is built, so WC51 can do its work as good/bad as it does now. As a player who somtimes like to go off meta, and even use this piece, i can say, that it need to have a bit more armor - to be at least on par wih UC, and on most part better pathfinding, cause now u can use this Wonder of Murcian engineering only in open maps without many pathblockers.
Tip on it - use it with rifleman inside, and hunt down alone enemy squads (or kubel), never engage more than 1 squad (or 2 volks), or u will lose, main purpose of this strat is forcing your enemy to blob up + building expensive raketen unit, thus giving you early game advantage, after 2cp call in Engies and set up few demos, cause enemy will invest in schrecks, and not in minesweeper. After that, if u havenet forced him off, just hope u have managed squeeze off everything, cause your late game will consist only from ELITE units, like rifleman and sherman.
But about changes - price is k (if u use refit ability) - if more armor and beter pathfinding (maybe speed) would be added