@John M. I think the deal is a no-go if PMI is for the life of the loan. The drag on profit is just too terrible. Save for a year and then try again.
To increase your available funds, can your roommate pay 1 year up-front for a discount? Do you have a Roth IRA or 401k from which you can borrow medium-term funds? (Borrowing from your retirement isn't a very good suggestion.)
Regarding your offer, maybe initially I'd try $165k with buyer paying all closing costs, seller provides 24 months @ $10k warranty for the roof (funds held in escrow at title company), buyer puts 5% down, seller takes 15% 2nd mortgage (rate 5% @ 5 year term). This offer will not be accepted but hopefully will garner a response. Hopefully they bite on the 2nd mortgage which would allow you to avoid PMI.
For the 2nd offer, dump the roof warranty. Increase offer to $172.5k with seller providing $5k towards closing costs. Note the switch from buyer paid closing costs in the first offer to seller paid closing costs in the 2nd offer. The specifics of the offers can probably use some tweaking but the point is to start with enough variables in the first offer(s) so that you have room to negotiate later.
One last tip. Before you are serious with your offer, set a max amount for yourself. If this is business then there must be a point where the deal doesn't make sense. If you don't have that preset price then, invariably, their "final" offer will be $5k yours. Then you'll capitulate because you're emotionally invested in the deal.