A 20"f4 Zambuto mirror is about 9k plus shipping ( there's an 18f4 Quartz for sale on Zambutos site right now 6.5k US) . Secondhand 20 incher SDMs pop up on IIS occasionally with plenty of fruit for around 10k.
I would either buy a premium mirror and then worry about the scope, ie: build myself or buy a dob carcass sans mirror - or wait for a secondhand 20 incher with everything to pop up in the classifieds.
I have some dobs
