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Ballpark number - $30-50/SF for normal garage - in your area, considering need for perimeter foundation to get below frost depth, probably about $40/SF unfinished, $50 finished.

In your area, if you can afford it, I would talk to a Realtor friend (or the one who sold you the house) about how much value a garage would add, and the difference between one and two car - very few houses are built with one cars these days except on narrow city lots, so it might pay (if your budget can handle it) to make it a two-car while you are at it.

Also, a standard 1 car is about 12 x 20 feet - if you make it large enough for a full size pickup or SUV then about 12 x 24 or 12 x 26', which could improve resale value. The current trend is toward 8 to 8-1/2' ceilings and 7-1/2 to 8 foot high entry clearances also, rather than 7 to 7-1/2' ceiling and 6-1/2 foot beam clearance, to handle larger pickups and SUV's with racks.

Definitely - if you are interested, there are good videos on how it works on This Old House and Hometime websites, and probably on Youtube too, or google  -  "epoxy floor covering"

It takes A LOT of careful prep and you have limited working time to handle it (it is a 2-component product like epoxy glues and fibreglass resin, so has vdery limited working time after you mix the two components), so unless you are quite experienced with fast-setting finishes I would NOT recommend you do it yourself - get it wrong and it is MUCH harder to remove than to put on. Also, it is important to sprinkle the non-slip material over it at the right time without stepping in it or getting any on flooring that is not yet covered with the epoxy.

Find a specialty contractor for garage floor sealing - normal painters and contractors may or may not know how to do it. Rough cost estimate, WITHOUT moving a lot of stuff in and out of the garage, $1.25-3.00/SF depending on how expensive a finish you get. Keep in mind you will have to move everything out of the garage for a few days to a week, or pay them to come twice (which I have seen done) - to do it by halves about a week apart, with you moving workbenches, tool chests, etc from one end of the garage to the other before they come back.

There are a lot of off-brand products out there - a lot of them are garbage, and some claiming to be epoxy are not even 2-component, so not true epoxy. I would stay with national industrial brand names like PPG, Rustoleum, Armorchip, AkzoNobel, Dulux, ISF - the ones made by consumer paint companies like Behr, Sherwin Williams, etc seem to have a lot of bad press about them.

One thing to consider, is if your slab was coated with a water repellent or sealer, that will probably have to be sandblasted or ground off before an epoxy can adhere properly - for about $0.50-1.00/SF more.

If you are not up to spending $1000 or so on protecting a floor you drive on, you can get Rustoleum Epoxy Shield Concrete Floor Paint for about $40/gallon that, as long as you degrease and scrub the floor well before hand, sticks quite well including over a normal concrete sealer. While the rustoleum stuff is actually an acrylic, not a true epoxy, it does good for the $. The competing latex concrete paints peel and lift off in no time - they are fine for coating a basement floor to reduce moisture passage before putting down flooring, but do not hold up to garage traffic.

I redo the central driven-on half of mine for about $45 in degreaser and paint about every 10 years and while it does not look like a showroom floor after the first few weeks, it does the job of being easy to clean and repels most vehicle stains (except gasoline, paint solvents like laquer thinner or acetone, or automotive spray solvents like brake or carb cleaner, which will all soften and bubble it). It also is not anywhere as slippery as an epoxy finish. You do have to let it dry a full week to avoid peeling from tires sitting on it.

As a condo owner I presume you have checked with the condo manager - most condos have rules about types of finishes that can be used, and in some cases even colors.

That is a pretty old and small house to be putting much money into.

My recommendation would be to do no more than box in the carport as a garage, if you have use for that.

Adding a bathroom and laundry room to a garage generally means a new foundation for them, so significant cost for the small added space you are getting. Generally, one would recommend an addition rather than putting those in a garage. However, for this old a house an addition could be equal to the value of the house.

My recommendation - talk to the Realtor you bought it from, or a Realtor friend, about what certain improvements could do to the value of the house. I fear you will find that anything other than a teardown might not add to value at all, because any significant addition to that old a house, and that size if typical of the neighborhood, would make it an oddball house for the area and reduce rather than enchance saleability.