[TheReferee]

seems to me you have covered all the options. if you dont want to buy new lipo's then its the X6.
if your not confident about your own setup ability then again more setup info on the X6.

if you dont mind new lipo's and you are happy to work on setups then C4.1 or vega.

if you want a proven national winner, then its X6 or C4.1.

other considerations are the amount of spares you already have, the X6 uses more of its own parts than other B4 conversions.

As we pit with Tom yardy and i have seen the development of the C4.1 over the winter and into the start of the nationals i am of course slightly biased towards that car.