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.

New wiring and plumbing, bathrooms, and kitchen were all part of the plan but we also ended up restudding almost the entire thing because it was in such bad shape.

You have plumbing and electrical in that wall, plus if you tear it all the way down, you will have to redo the floors. 

Especially because sub-contractors were able to point out items the GC’s overlooked, in particular with plumbing & electrical.So I decided to proceed with being my own GC however hit a road-block with the permits.