There are plenty of helpful how to videos on you tube to guide you.  

Some of the things you'll want to watch out for are knob and tube or aluminum wiring, lead paint, pipes that are extremely old (cast iron drain pipes can last 100 years but many types have a shorter lifespan), when was the roof last repaired, how old is the hvac, asbestos, old chimneys or stone foundations sometimes need to be re-pointed. 

The plastic tubing on most refrigerators is cheap and easy to spring a leak.

That's a glass tube with a cheap plastic rose in it.

(old widow house past away) this may be a nob and tube house in my area that is not grounded. our neighborhood age and my guess on this house I would plan a remodel that allowed me new plumbing and electrical.

However, I’m wondering how cautious I’ll need to be with buying homes 100+ years old, or if I should avoid them entirely.The issues I’m most concerned about addressing are:-Knob and tube wiring-Foundation issues-Plumbing, cast iron pipes, galvanized piped-Lead based paint-Asbestos-Old single pane/drafty windows-Old HVAC systems-Roof-Historical preservation rulesI intend to leave the work of fixing most these issues to electricians, plumbers, roofers, etc.

The house has had many self (non professional) add-ons since it was built in the 70's to include a sauna and hot tube in the master bath area (originally a 1 bed 1 bath I believe).

While they can offer great returns, if previous owners haven’t replaced older systems, you’ll need or you will spend considerable more in routine maintenance and capital expenses both. from cast-iron pipes to electrical (Knob-and-tube wiring (sometimes abbreviated K&T) to foundation issues, I’ve seen it. 

I found 2 videos on U-Tube in which info. is presented, for this possibility, & one is by a British guy, whose last name is McCoy-Ward, & the other video on U-Tube of this theme, is by an U.S. man, who calls himself, Marko White Board Finance.

Although, it usually is a critical item to sell a house at all and can keep people from purchasing if the plumbing is old or electric is insufficient or knob and tube.