I bought my m6 vy sv8 pre-covid 2.5k still registered was suppose to be donor car for vp ute v8 conversion but ended up becoming a daily for a few years.
Doing all the work & extra mods I wanted reckon it would have just been under 10k if waiting for right 2nd hand parts to pop up on marketplace,gumtree,forums etc
Got lucky finding reco vs 304 stato driveline at Pick n Payless for $1100 from radiator to tailshaft that was up & running maybe $1500 that quickly doubled when added a few choice mods...again pre-covid.
All depends on price of donor car or driveline then waiting on 2nd hand parts you need for conversion & whether or not you can do all work yourself.
20k would be a safe if buy ls1 stato 5k-8k donor car...lots of little things add up moreso if have to pay for labour.