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.

I usually charge an additional  $ 50/month for a pet with a stipulation sometimes they have to have a professional clean the HVAC ductwork upon leaving (Pet hair).

I was already down there doing some duct work repair, so it's a great idea to turn on all the water sources and check the drains.    

Installed Ductwork and HVAC

You could run the lines up the interior of the wall and down to the handler and have a plentum on top, but your duct work will run through the attic, but I don't think that would be a problem.

Asbestors can be found in old boilers, duct work, attic and insullation, floor tiles and rolled flooring, old carpet pads and in plaster and lath.

So, if you pull an electrical permit to move some wiring and the inspector notices that the plumbing, insulation and duct-work aren't up to code, he can require you to fix all those things as well.So, if you plan to pull any permits, you should be budgeting to fix anything in the house that's not up to code, just in case the inspector calls it out and requires you to fix it.As an example, I finished a property a couple weeks ago, called for final inspections and the inspector required me to replace the bathroom window with a tempered window.