Off market opportunity - need advice

[Joe Coyne]

I've got an opportunity to purchase a deal off-market, fresh out of probate.  I know the property quite well as it is next door to a rental property of mine.  The new owner of the property (seller) seems willing to do this without a realtor, as am I, mainly so I can justify a lower bid.  I also don't want a listing agent coming in encouraging him to list much higher than my calculated value.  

Am I over-simplifying this by thinking I can write up my own purchase agreement, with contingencies, and then get my broker to initiate underwriting?  And then between the lender, title and the buyer + seller we can close the deal?

What are the downsides here?  Should I be going about this differently? Should we mutually find an attorney or agent to help facilitate the deal?  Any advice would be appreciated.  Thank you!