I am currently throwing money down the drain by paying $850 in rent at the moment, and noticed that I can be paying for a mortgage for almost the same price.

When someone throws out a number people often use it as a reference point even if the number has little value.

I'm just trying to throw out some "out of the box" suggestions.

Buy the lot, split it into 4 lots and buy some trailers and throw them on it and rent those out.

I think you're ready to purchase a tax-deed the major down-fall is that too many foreign investors and small capital groups come in and buy the best properties near or above market value....There's a bunch of posts on tax-deeds its just you have to spend sometime  learning how to title examine (this is a key skill you will have to learn).1/2) These properties are being "foreclosed" by the local government - in general its state specific for instance in FL it is true they can be redeemed until the second before sale however after you're pretty much done for (however if its a valuable property and any of the previous owners in the chain of title have a bit of cash to throw around you'll be forced to defend yourself in court which can be pretty expensive).

Likely the reason why you got this off of a tax deed is that the prior owners determined they couldn't do anything with the land either and quit paying the taxes rather than throwing good money after bad.

A zero-money down deal on a 4-plex sounds freaking amazing.I hate to throw cold water out there, but I would question the validity of the deal. 

There is often a way and just throwing up your hands and giving up is rarely the only thing that can be done.   

Meanwhile he was throwing me offers or “opportunities” to invest with him.

I know many people throw a barrage of questions to shoot down an idea.