I don’t no if this will help but this is how we negotiated our last property that needed a bit of work / renov to it.
As you mentioned rough asking price is appro $320000 and as Rikky said before -10% then work out approx cost for renov”s and take that from the asking price as well.
Are your finances in place ” have pre-approval” etc if not get them in place.
Do you have access to some spare cash ?? maybe $10000
Once all this in place go back to the vendor with a writen offer with something like this:
Offer : $305000 / $310000 with a $10000 cash dep “and wave / or hand them $10000 ,quick settlement 10/15 working days offer is valied for 24hrs.
Worst case they say no.[blink] but alway keep in mind your profits are made when you buy the property.