I've thought about getting licensed but I just don't have time for that right now.The main drawback is the time and expense, both getting your license and then hanging your shingle with a broker.  

Eventually we had to re-deck the roof not due to pests but because I needed shingling and they could not re-shingle to the planking, with slate you may just be able to repair the slate.  

We had a new roof put on the building and of course it needed all new wood under the shingles so that went over budget.  

Are those shingles just a little old or are they completely worn out?

The biggies that you don't always see (read expensive) for me in CO are stable soil/foundation/water intrusion, aluminum wiring or out of favor fuse panels, hvac if not replacing, sewer scope in most older neighborhoods, roof (which will need to be brought to code (shingles, ice dams, re-sheeting maybe) on the sale usually if it's older). 

But I think you're mostly looking for properties that have lawns that haven't been mowed, overgrown bushes and trees, missing roof shingles, piles of newspapers on the porch, boarded up windows, broken or missing shutters and gutters, and For Sale By Owner signs since sometimes those won't be on the MLS.  

Oh yeah, 20-year shingles too. 

(Nice architectural shingles on the roof, etc.)

  $20,000 should still be able to pull off the shingles and have it re done for less than that!

I think a few shingle replacements in the summer should stave off anything serious for a while.