As a former electrician I can understand your predicament. First knob and tube KnT as long as not modified or compromised is not a code violation and should not pose any issues with getting CoA’s, it will however likely result in higher insurance rates if the inspector decides to dig in the attic or behind insulation in the basement or junction boxes in the ceiling. Also since power consumption rates have increased over the last several decades this installation type may not be up to handling the increased load reqmts. Since KnT hasnt been used since the late 30’s early 40’s you can be assured the cotton and rubber coverings are as brittle and dry as a cracker and more like kindling than wiring. Quick tid bit: based on the NEC ntl electrical code NFPA 70 KnT was allowed at the time I was practicing but rarely if ever used residentially due to cost and installation overhead, you see it everyday however in the design of power transfer lines that criss cross the country. With that said you need to evaluate safety, performance, cost and future needs before diving in. Not knowing specifics or geographic location or power needs for this home and using basic code requirements a total rewire for a 2700 sf home with new hardware and trim excluding a new service would run somewhere in the neighborhood of $25,000, this does not include the repair to exterior surfaces that would be required to “chase” wires across beams and studs. In usual KnT installations the house was built with a balloon frame which means that the interior walls were open from the ground floor to attic allowing a straight run for wiring and pipes but this also allowed for fires to spread quickly and were outlawed per building codes. If the walls have “cats” or fire stops the time and labor will be increased add onto that the difficulty associated with running wires through insulation and the hassle of plaster and wood lath walls which were also the key building materials at the time of KnT your costs may increase. You can do some simple math to gauge potential costs, if you know what master electricians charge per hour in your area and add to that the hourly rate for 1 journeyman and 1 apprentice and multiply that by 80 hours you will have a rough idea as to labor costs. I estimate 2 weeks not knowing the structure but roughing and finishing new construction which is about 10 days I added a few days to account for the roadblocks that will inevitably be encountered. You can estimate material costs to be 30% of labor total however with copper wiring having skyrocketed the past 5+ years and new code requirements pushing costs higher this may be on the light side. I guess what I am trying to say is that this will not be a small undertaking, it will not be cheap and if it were me, a guy who could do the work at cost, I would consider using the cash to buy an updated property and put extra cash towards the things that increase curb appeal and rentability and leave the rewiring to someone with a passion for self inflected pain. Good luck on whatever you decide.