This tool (no affiliation with BiggerPockets) can help your son find properties with assumable loans. 

I will give you my numbers since we are 3 years into Velocity Banking.Home was purchased at $410k.Interest rate: 3.125%30 year termMortgage payment is around $1950 (includes insurance and property tax)HELOC: 8.25% Variable rate - current rateCredit Limit: $97kLet's say I made $10k a month and our expenses were $6k each month including the mortgage.

Purchase price: $105,000 Cash invested: $90,000 Property needed some serious rehab, from trees roots destroying the basement walls, to plumbing, electrical, to roofing, landscape.

San Francisco has a higher property crime rate. 

Over the last 3 months, I’ve moved 4 of our 12 units over to a property manager.

Capitalize renovation costs to increase the property’s basis and reduce future taxable gains.

As far as benefit to the seller goes, @Matthew Paul I think you are probably right on the money and regardless of whether they would have preferred to stay in this house rather than move, it is likely providing them some excellent cash flow; and I'd be hard-pressed to come up with a deal that serves both parties better than the current situation. 

Quote from @Akshay Kumar: Hello everyone, I am a new investor and looking for advice on my first investment property.

Max of three rental properties.

If you are living there you can keep the 4% rate.If you are not living there you can rent it out to pay for the property taxes.