Our selling price is 172k and the amount of work wanted sums up to about 6k+ (laminate flooring in kitchen and bathroom, new plumbing, new appliances).

We pulled an electrical and plumbing permit. 

If you have a plumbing issue I'm sure you will take care of it however if you have tenants that don't understand how a shower curtain works they would need to cover the damage that they caused.

What's a fair price these days for some basic rehabbing - not a full gut (roof/flooring is fine), but just:- Removing all existing drywall- Replacing all electrical- Replacing all plumbing- New drywallBasic 3/2 1200 sf, one floor.

It needed all the main systems replaced Electric Plumbing and HVAC.

I would look at all the major expenses you’re likely to have during your hold period, (Roof, water heater, driveway, Sewer line, furnace/HVAC, paint and carpet/floors every turnover, appliances, etc. depending on how old each of those items are and how long you plan to hold, plus few thousand here an there for that nightmare tenant who ruins everything, the hail storm, the plumbing leak that happens when your tenant is out of town and turns into major flood damage, etc. it adds up quick) total those up and divide by the number of months in your hold period, there’s your monthly budget. percentages are garbage on these low rent properties, easy to get in trouble thinking any percentage of a low gross number will be adequate. 

And I have 27 yr of experience in plumbing, carpentry; elec, remodeling etc.   

The plumbing was $6100 for the whole house 

2) How does one typically get comfortable with plumbing and electric for building which is 100 years old?

I was always wise with money but around 21 I really started hammering down and by 24 I owned a mixed use property with a rental, my own place to live and a plumbing business.