Congrats, in advance, on your new home.
A few tips...
Make sure you have a full home inspection done. Duh, you probably already know that. Shadow the home inspector with a video camera or take pictures. If the inspector doesnt want you to do this find a different inspector. Don't go on the roof with him but at least into the attic and crawl space.
Make sure you understand all the possible issues. Adding AC is nice but if you have a leaky roof, unstable foundation, unsafe electrical, leaky plumbing, etc. Then those should be a first priority.
As for adding AC - I have done duct work myself before. Both times I was working alongside or with a professional. first time the pro did all the calculations, designs, etc. and made up all the duct work (ridged and insulated) in his shop. He came on-site and showed me how to connect the pieces. He did the main connections at the furnace. The second time I did it I was working with a pro and we installed a combination of fiberglass and flex duct, that was specified by the 'builder'. (after katrina, doing some relief work). We discussed the pros and cons of fiberglass and flex ducts versus ridged. Yeah, fiberglass was easier to fabricate onsite and a lot cheaper. Ridged gives you durability and max airflow.
Anyway - yeah its possible to DIY duct work but work closely with a pro and pay for his time and expertise. Also consider possible one or more mini-split systems. They don't require duct work and are easier to install.
You do have a thing hanging on the wall tho. Usually up high. They are much more quiet than a window unit. They are typically a professional install but pretty easy for any pro with the right tools and experience. They can come with one compressor (outside) servicing multiple inside units. You may be able to get away with one or two inside units if you have an open floor plan.
Okay, I'm rambling. i have a DIY home inspection checklist. DO NOT USE THIS AS YOUR HOME INSPECTION. But, listen to the show and print out the checklist and take it with you, if you can spend some time at the house check some of the things on the checklist. It will give you an idea of whats what.
both the download and the show where we discuss it are here
Episode #57 – Springtime House Inspection