First deal under contract, inspection issues on as-is sale

[Andrew P.]

I'm under contract on my first deal.  A backdoor duplex (not zoned as a duplex) in a hot market.  It was built in 1978 and I knew it would need around $20k-$25k or so of work.  Under contract for $180k and rent is around $1000/mo per side.  The seller negotiated but made it clear that this was an as-is contract with a due diligence clause.  My inspector was in the crawlspace and noted no vapor barrier or insulation under the house.  It was damp down there with some surface mold and random plants growing.  This is probably another $10k-$15k to remedy.  Also they said the whole house was PEX piping but one side is still polybutelene.  With that said I want to go back and renegotiate as there are more issues than expected.  What is the best way to do this and be successful?