Rotten joists- economic?

[Debbie Quinlan]

Hey. 

First post here so mea culpa if i'm posting in the wrong forum! 

I currently own one rental that I used to live in, and now I have had to move to a different area. In this new area, I have a good job. I found a fixer-upper, for some 50k here. It is a 2 bed. The issue? Well the seller is very motivated which is great, but it has no basement (Crawlspace only) and the joists are rotten around the hatch at least (in the kitchen). I could smell mildew when I walked in. There is a huge soft spot in the bathroom as well and I suspect not just rotten joists, but also rotten floor. I gather that is where it originated, but listing agent is not sure. Land is worth around 18-25k around here, and the utilities are currently turned off. 

The roof has at least five years left in it, and I suspect that this would be a 'gut and tear out' type of job. 

I could not see any major cracks in the foundation in a walkaround, but clearly the water in the crawlspace (damp, not standing water that I could see) came from somewhere! We have frost/snow heave up here. 

I have found one other house on mls for 70k, and the rest all start around 120-130k in this area. Rental for a 2 bed house is around 750. I suspect I can negotiate down a bit more from that 50k and the mortgage would be some 250 or so a month (Depending on exact interest rate) at 20% down. Property taxes I'm told are around 2200 but i'm not believing that as that sounds very high for this property. It has sat vacant for at least a year. I am not unhandy, but I have also never done floor joists! (laminate, tile and the like I'm all comfortable with), so I would need assist/contractor for part of this. 

I know I /can/ fix anything as if it can be broken, it can be fixed. The question is, how much would I theoretically have to budget for this kind of thing, and how bad a case am I looking at?  The place is listed as 775 square foot and quite frankly, I suspect the entire floor would need replacing.