Adding central air was a major expense we hadn't counted on - and though we got a good deal on it this and a few other things meant we were a bit over budget. 

I would like to renovate the kitchen, repaint, and furnish and equip the unit, so I am setting my max budget at $50k.  

If people are complaining about CAPEX, repairs, and vacancies eating up their cash flows, they weren't budgeting properly for expenses that EVERY rental has. 

(For example, many investors talk of doing rehab on a 50k budget--but then on the same project, many GCs will charge that much for the kitchen alone.)Is it because the former buy materials themselves & have subcontractors install them?

You can eliminate marketing costs at first by talking to everyone you know and ask them if they know anyone who needs your service.Also, I would recommend having a marketing budget even if that budget is not large.I would suggest starting with 500-1000 names per month and setting aside that money for 3 months with or without a deal.

I would typically budget 5% or $135 per month for vacancies, $250 per month for repairs and capital reserves, and you have 8% or $216 for management for a total of $601. 

Knob and tube wiring, leaks in plumbing and it was the very top of our budget so no $$$ for repairs. 

You should establish a target budget and timing and have him work toward those and track closely.  

, who gets the budgeted amount for vacancy?

They meet the second Wed of the month.