When the son eventually sells the house, does he have to wait 2 years starting from May or November to get the 250k primary residence capital gains exclusion?

So you will break even but eventually (after 30 years) will have a fully paid off house. 

Eventually, I thought there was too much risk (market saturation) and I chose another STR market.

I took a partial withdrawal of $42,000 from my 403B plan five years ago to buy  a forclosure for $50,000 in cash.The property has been rented to the same tenant for almost five years now and I have collected almost  $50,000 in rent.Today the property is worth  $135,000 and I took a Heloc on it last year to buy two duplexes.I didn't mind paying the tax because I would have to pay it eventually anyway,  and the penalty of $4,200 I got back in five months from rent.

I would need to probably include farm profits in my calculations because the hope would be to eventually farm full time, but also supplement current income till I can move to farming full time. 

When you eventually get a deal under contract and only have one lender if the lender decides not to do it then your stuck. 

I intend to do rental arbitrage to start and then eventually purchase.

So if it comes to starting out like you said, I do believe single family 1-4 units is the way to go until they can eventually build up to get to multi family. 

Some people get lucky and enter the game when everything is easy to sell, but the market eventually dries up.

Although right now is a fairly tumultuous time to get into the space I would do what you can to dive into education as to how to evaluate deals and what you eventually want your portfolio to look at both in terms of composition and risk profile.