Wiring, plumbing, Windows , and construction is going to be old school and the value is just not there. 

I budget for maybe $2000/year... again, that's a worst case scenario, but, particularly with these older homes, things will come up (not sure how much of everything you are replacing with the $25k, but, every single one of my properties has required some type of expense that I would consider non-trivial ever year: boiler breaks, central air conditioning breaks, roof leaks, bathroom and plumbing issues, electrical issues, etc.). 

I was looking for more signs to watch out for like Hvac or roof or plumbing.

Get familiar with the cost of everything: i.e. cosmetic fixes: kitchen, bathrooms, flooring, tile, painting, base boards, roof, electrical, plumbing etc. 

I have many years’ experience in different aspects of real estate and started my professional career as a plumbing and heating contractor right out of college.

Demo, electric and plumbing, patch and paint, carpet, tile and granite.

I've been a GC for 30 years, I have made deals with first time customers where I break a job down into various aspects, i.e. plumbing, or drywall, flooring e.t.c.. where I get paid after each aspect is completed, that way I have some cash and the customer doesn't have to give a big down payment up front, win/win situation.

He reported plumbing issues and some insect control problems (roaches...not uncommon for a house that rents to college students).  

I'm doing another one now and with some unforeseen plumbing issues, I have an extra $1k in rehab...but I can afford to pay that so it's ok and I'm not stuck.

I think this ones going to need to much work, foundation, plumbing, etc.