You may need to pay more for a more stable or trustworthy contractor that can either bill in arears (like a professional) or can be trusted with an retainer (like a professional) to go purchase... 

I have accumulated some pretty good sub-contractors over the years (painters, plumbers, electricians, dry-wall), but have been struggling with GCs for permit pulling (when needed) or carpenters for small unit turnover projects, large building renovations, or even small isolated exterior fixes.

For example, you buy the place and the tenant could claim the Seller allowed them to paint the walls black or that their security deposit was twice what the Seller claimed.

Structural issues, terrible smells, no HVAC, Plumbing, electrical, walls, ceilings, etc... wood rot, termites, asbestos, you name it!

So now you retain a REI Realtor to help locate and get under contract cash-flowing deals with good equitable gains outlook, and put that capital to smart use. 

For example, you make a hole in the wall to install new ducts or a hole to access plumbing.I see prices per sheet hung + finished but not for smaller patches.

My work has taken me from leases on Wall St in NYC to small industrial leases in Joliet, IL and beyond.

Great for trailers without built up walls. 

What kind of retainer/hourly rate do they charge?

I have contacted several attorneys and they all want a couple of thousand dollars as a retainer.