I will not recommend on compromising on the cost of the Idler.it is in may ways the heart of the TT along with the bearing. even a mm change in dia will affect the speed.
additionally the bearing for the idle should also be of good quality ie frictionless and with absolutely no wobble.
while you can play around with the plinth quality, the Drive ie Motor+belt+Idler+Bearing all have a very significant role to play in a TT..please do not discount that.
I would love to hear the view of Rajiv here since he does have Thorens as well.
my suggestion is to Bite the bullet and guy a good one
Thorens idler wheel for TD 124, TD 124 MkII, TD 184 etc. BRAND NEW !!!! | eBay