I've done this for a cottage and a garage loft on the Cape and two second floor additions for cottages on a lake in Maine. First make sure the upper floor joists are adequate. they are usually sized to only support a ceiling. Measure the span, spacing and size of the members and let us know. Then you need to draw the floor plan carefully to scale. For the stairs, measure the floor to floor height in inches (including any flooring changes), divide by 8.25" then round up to the nearest whole number of risers. Subtract one from this number to get the number of treads. Multiply this by 9" and convert to feet & inches which is the total length of the stair not counting the landing dimensions top and bottom. Assume the stair will be 3 ft wide and the landings 3ft x 3 ft. It is also possible to use an alternating tread stairway which cuts the stair length in half but might be difficult for small children to climb. If the upper floor area is under the eaves of the roof rafters it is a good idea to put as much of the stair as possible in the low ceiling area and have it rise as the roof increases in height. That may be difficult to do on the lower level but give it a try. It helps if there is a hall or entry foyer available so you can enter the stair without creating another hall space. A stair that turns 90 degrees half way up sometimes fits better in the lower level plan but it takes up considerably more space on both floors. Beadboard paneling is available in 4x8 sheets for paint and in 3/8" thick t&g pine strips for clear finish.

Your best bet is engineered roof trusses. I think you will find the cost much cheaper for stock and installation. Most lumber yards can assist you in proper sizing and any other considerations. The only caveat is the loss of some attic space. I can tell you for fact 3 guys,4 hours, framing and sheathing done. Make sure you are thorough in your objective(drawing) when you go to the lumber yard.

We did it in our old Victorian house for pennies. We turned a closet in the bathroom that was behind the tub (!!!) into a laundry closet with a stackable. Complete DIY job and I would definitely do it again. Our only "problem" was that the pipes up there couldn't handle the flow of water as fast as it came out of the washer. So the draining washer water backed up into the bathtub, which filled with about 6 inches of dirty water during a load until draining out. It never caused the tub to look dirty or left a ring or anything. We just lived with it as a quirk and I absolutely LOVED not having to lug clothes up and down to the yucky old basement via the yucky old non-standard basement stairs. The draining is something to really understand before you start demolishing your setup. We had no problem selling that house--everyone wants convenient laundry access and no one cared about the draining into the tub thing in this Victorian town. The hardest part was getting the stackable up the curving Victorian stairs. My husband and his friend almost toppled and killed themselves doing that. Just make sure you have a disaster plan in place--as for any 2nd floor laundry-- if something breaks or overflows, you don't want water all over going into the ceiling of the floor below. Use the best fittings that last (no plastic), the tray, etc..

Your max is the lowest bottom end potential cost. It will more likely be 30-80% more than that for something average in build quality. Depends on if you are in Kansas City, MO, or Alexandra, VA. That is if the foundation and garage framing can hold up a second floor, and the top plate heights at all line up with the home. And the home's main drain is on the same side of the house. Most likely one of those assumptions is not the case. You might have to tear down the garage and completely start over as a new build. And pay for a new sewer tap. If allowed. If the main drain is in the wrong spot, the project will never be feasible to do.