When I run the numbers I seem to get expenses (including vacancy) that is usually 55-60% of gross rents.  

I went out on my first real Driving For Dollars trip looking for properties to wholesale and came across a neighborhood of old and run down mostly manufactured homes (on brick).

Personally I would call a few banks tomorrow and find out who can do a cash out refi, and get their criteria run your deal by them.

As far as whether to hold/sell/etc your land, I suggest running the numbers for specific cases and comparing the options.

I am already using LoopNet, but I think I need an actual broker that can run comps and help me analyze deals.

If you can't, because the seller won't allow it, then pay your own title company to run title in advance before you plunk down your deposit. 

I'm supposed to sit down with the accountant/mentor and run through a couple of scenarios to figure out what I want to do.

So I would expect for the seller to tell you to take a hike, and you wont have to worry about your offer.Regardless of the repairs costs, unless you have reason to expect the appraisal came to the value in error (bad comps, errors in the specifics of the property, etc), than the $340k number was never going to happen.Sounds like the property has multiple problems, and it sounds like your not an expert at spotting these and have to rely on contractors, I would recommend you run from this property.  

I've run across a property that has two 4 unit buildings on one property. (8 total). 

The list had over a thousand properties most were junk but I found two that were worth buying and both turned out to be home runs - beginners luck.The one most important rule is know the value of what you are bidding on.