First off congrats on finding a house that works for you. As someone who just went through that process 6 months ago I know how difficult and stressful it can be. I would simply advise getting estimates on all the work that needs to be done. We made some of those updates ourselves, updating the electrical was about $2500, that was just for putting in a new board. We also had to put in new outlets and switches in the entire house but I did that myself. In the wonderful older houses that we all live in there are bound to be some "creative" solutions used over the years. As long as everything is certified as in safe working order the homeowner probably won't pay for any repairs but it never hurts to get an estimate.
The roof will cost you but without knowing the size of the house just how much is anyones guess. The homeowner probably won't consider paying for this since the roof does have life left. However if there are any leaks the homeowner should make repairs.
Having to replace the furnace and AC might actually be a blessing in disguise, since you will be able to select the units. Our stuff was all new but the homeowner was going for the cheapest possible so we have units that work but are not energy efficient and slightly undersized for the house. Once again this is a considerable expense. The homeowner will most likely not be footing the bill for this.
Slow drains need to be looked at! We ended up having to have a plumber clean our main drain all the way out to the street, the bill was still only a few hundred bucks but that is something you might get the homeowner to give you a credit for.
Radon level, I know nothing about but I did a quick search on line and it seems that you can get a vent installed for about $300 lower that number
A Citizen's Guide to Radon | Radon | US EPA. This is something that you might want to try and get a credit for.
Termite damage is nothing to worry about as long as it was treated and there is no current activity. However damaged areas should be examined to make sure that structural integrity has not been compromised. Any structural repairs should be the responsibility of the homeowner.
All in all, I've learned that everything in a house can be fixed. The one thing you can never change is the lot and location. If that is good that is
by far the most important thing.
Other expenses you can count on are paint. Each room, depending on the size, will take between 1-3 gallons of top quality paint. You will probably buy the most expensive paint( Benjamin Moore Aura), like I did, which is about $45/can. Window treatments! OMG I had no idea. We even went to Blinds to go and Selectblinds.com to save money but still dropped almost $2K on them. Floor refinishing, we thought they were fine but all the scratches really ended up bothering us so we stained and refinished for about $2,500. General home depot runs will run you about $200/wk for the first month or 2 in the place, that might even be a little low and doesn't count the cost of Lawn Mower, Snowblower, rakes, and other lawn care stuff. Also be advised contractors, handymen, plumbers and electricians are rarely on time, hardly ever return messages and can really only ever be counted on to show up when you owe them money. Hope that helps
Best of luck in your new house, I have found it to be stressful but really rewarding and definitely worth it.