It would be my primary residence for at least a year, so I'd want it to be nicer than a standard rental rehab i.e. hardwood floors throughout (maybe bamboo to cut some of the cost), exposed brick walls, 1.5 baths, subway tile in full bath, central air/heat with exposed ductwork, rooftop deck if city approves & budget allows (not a necessity though).

A furnace should be elevated off of the ground when possible but there should be a crawlspace under the house where you can run duct work.

If i decided to purchase this property and started to dig into the walls and found asbestos wrapped ductwork, galvanized plumbing, and knob & tube wiring....i would be upset if i didnt have a contingency that accounted for potential problems.  

So for instance in our project now we had the plumber doing the rough in, the HVAC guy installing some new duct work while the crew was demoing an addition and adding siding, etc.

I then start adding other items such as replacing windows (250 each), roof (5K), HVAC (2500 to replace a unit, 5K to install unit and duct-work from scratch), etc, etc,.

Ultimately, it depends on what your competition is doing.I will say that central HVAC certainly seems to add a lot of value (more than the cost) based on what I've seen in my research.The only other thing I would be careful about is lead paint if you've got to cut into walls and add duct work.

There is no Duct work exist; its full rehab; including wall and ceiling and windows replacement; its been not maintained well and with same owner for around 40+ years.

I have a similar 1926 Spanish bungalow in LA (3bed 2 bath, 1300sqft, single level), that i just rebuilt completely, so here's my 2 cents1. with plumbing and sewer, make sure that you insect and assess the main drain line form house to street sewer - in houses of that age, they are cast iron and are often buried under a concrete driveway. if it needs to be replaced, make sure re-pavement of driveway is accounted for2. houses of that age have often started as 2bed 1bath and then somewhere along the way been converted to say 3/2 - mine was in the 70's. ask whoever inspecting the foundation to pay attention to the way it's laid out - variation in crawl-space hight, floor joists and different materials used for pillars that house sits on could point to an addition. flat roof over one of the bedrooms or bathrooms in an otherwise house with an attic is another sign it's been added. either way, check with the city that those additions are legal/permitted3. termite damage and dry rot in California and Arizona is a given - in any house, especially that old. as you walk around the rooms, stop in each corner of every room and jump up a few times - if the floor is soft and even a sign of weakness, i bet you you'll have to re-drywall, remove subfloor and repair floor joists in at lest half of that room. just a simple "poking wood for termites" test from the crawl space often misses that. i like my jumping test as well5. size and age of electrical panel - yes, but also age of main electrical wire runs is important. those are usually suspended in crawl spaces or laying on top of ceiling joists in the attic. old wire should be replaced as it's a potential fire-hazard surrounded by all the old fluffy insulation that's in old attics - speaking of fluffy insulation, i hear in some cases it could contain asbestos, so wouldn't hurt to checkfyi, just installed central air in my house and my best estimate - had 7 total - came through Costco at just under $7000 for a 3-ton Lennox system (heat and cool), all new ductwork, permit and inspections. curious as to what your HVAC bid was

With a ductwork system it's a little different than say and eletrical that would need upgraded.

I heard that they put antifreeze in plumbing pipes but theree is no antifreeze in heating ductwork.