There are so many variables in house wiring that predicting what needs to be done in a home I have never been in would be next to impossible. Same goes for the cost. Depending on the proposals the electricians gave you; the price spread can be believable. The best I can do is give you an idea of the things that might have been proposed, and what you should have.
The kitchen is your biggest problem. Basically, a modern home would have at least one 20-amp circuit for the counter outlets, plus separate 15-amp circuits for the fridge and garbage disposal. They would not be combined with any other part of the home. Generally, the 20-amp circuit would also be protected with a Ground Fault Circuit Interrupter, which is a safety system for wet areas. In our area, they are required to be retrofitted in older homes before one can sell it. That 20-amp circuit will also require heavier gauge wire. 18-amp usually uses 14-guage wire, while 20-amp uses 12-guage. When dealing with gauge, smaller numbers mean bigger wire.
You mentioned the stove was on the same circuit. This means you have a gas stove. Otherwise this would have to be on its own 40 or 50-amp circuit. When gas, the electric is used only to power stove lights and controls. Often it is okay for the power for a gas stove to be tied to the rest of the house. Sometimes you find it along with the fridge. It all depends on your local code requirements. Last, overhead lights in the kitchen can be mixed with other rooms‚Äô outlets and lights.
Since your house has so many things on one circuit, it must be old. If not, it must have had many additions, without the benefit of electrical code inspections. Electrical code variations depend on your locality, so check with your municipality. Also, you are usually not required to upgrade your electric unless you are remodeling, but in your case, what you described could be very dangerous.
A word about wiring. If your home is old, it may not have a third wire, also known as a ground wire. If it is really old, the wiring may be obsolete. In any case, running new wiring in existing walls is very time-consuming and costly. Your $3500 estimate probably included new wires, and no-doubt included steps to bring the house up to modern code. Your $800 estimate probably only entailed splitting off the kitchen from the rest of the house. Based on those assumptions, the range can be accurate. Make sure each estimate details the work to be done. The work should go along with what I have described. Your municipality is a good source for information on requirements and qualified electricians.
It really is up to what you want to do. For example, you could simply add an outlet under each window that you plan to have an a/c unit, and put them on a new circuit. That might be a good, inexpensive first step. Then, you can plan the rest as your budget allows. Ultimately, you are going to want to do more. Like you, I am amazed that you don‚Äôt trip circuits more.