Need advice on appropriate offer.. 8 units 2.4 acres in NY

[Devin Berrian]

My girlfriend and I are going on 5 years, and of course facing the big questions about tying the knot from family and friends. However, our motto has always been, "We're not getting married until we buy a house." (And by house, we mean multi-family home). Recently, GF's boss (a wealthy older gentleman with 30+ properties and several successful businesses) told her about a property he's looking to sell. It is on 2.4 acres of property, on a semi-private road, great school district, close to shopping but quiet, safe neighborhood.  He obtained it in a deal years ago.

The property holds one 100-year old farm house converted into 4 units (3-1BR, 1-3BR) and a 40-year old brick apt. building with 4-2BR units. All units except 1 are currently occupied, which would work because we are living with our parents in order to save up. The property brings in $100,500/year. The owner lives about 2 hours away so to avoid vacancies and issues he includes all utilities including OIL (about $17k/year), water, garbage, and electric. The net income is about $55k/year.

There is no price tag, no asking price, it's not even publicly on the market. The assessed value is $385,000, and we happen to know it's going to need a new boiler ASAP. Septic was done in the last 5 years and the income potential is there. 

The best part is that he has offered to hold the note. So I need some professional advice. After paying the taxes & insurance (~$1,900/month) and other utilities (~$1,826/month) and putting $1,000/month away for maintenance & repairs, what is a reasonable offer?

We would like to offer $2,500/month to cover a 15-20 year loan at 4%. I know that sales vary greatly depending on the area, so let me know if you would like to lend me some advice, and I can give you a closer idea of location.

Thanks in advance, BP community!