There is so much to think about when planning a trip, especially that first Europe trip when you don't have experience.
Look at a map and try to look for a logical route, ( either a circle and get a round trip airfare ticket or a line and a "multi city" ticket flying into one city and out of another) and keep in mind distances between places and if you plan to drive, fly or take trains. Plan to drop a car in the same country you rent it in. Think about the time involved in that transportation: getting to the airport early or train station and then getting into and out of each hotel. Most moves take at least a half a day or the better part of a day if you are flying. So, the more you move the more time it takes away from your sightseeing. Use viamichelin.com or google maps to see distances and debahn.com for train routes and times, skyscanner.com is a good site for flights within Europe. Then narrow down your desires. I think a minimum of 4 nights for large cities is good, and most other places 3 nights, very small towns 2 nights. As we travel more and more we find this many days are really a perfect fit. We are on the go all day, everyday, but it gives us a chance to get a real feel for the city/village and make lots of memories. If you go too fast it can turn into a blur and you don't remember much. If you have must see's for a particular area maybe you can find a home base and travel off of that. We love a trip that mixes countries, and mixes cities and countryside. It is a nice break from a city to take a few days away from the noise/crowds.
Most people that have not been to Europe have an idea that everything is close and you can see many countries on one trip. That is true if you stay in bordering countries, but not as easy as most people think.
A cruise is a great way to "SEE" a lot, but not a great way to "experience" Europe, IMHO. I cruised to Scotland, Ireland, France and parts of England (with my dad) but if someone asks if I've been there I usually say no without even thinking about it. I was there for a few hours but no real experience to hang a hat on, but I did see a lot.
We love a rental car to go as we please and explore where we feel like going, but everyone has a different travel style, so you need to think about what feels right to you.
As far as budget. That is easy if you do some research. Look at airfare costs, look at accommodations on something like Trip Advisor, VRBO for apartments etc. for a given area (once you find a place that looks interesting then go to that hotel/B & B or apartment website) and then RS has an estimate of what to plan for any given country for food, museums etc. Just look at his guide book for each country you visit. We plan more money for France and Italy for dinners than for Germany, so it can vary. We try to get our breakfast included as much as possible, we also prefer the small B & B's, but also rent apartments. I've also looked up restaurants online and read menus to get an idea of how much money we would likely spend. Look up Auto Europe for car rental costs. Look at the different maps for miles and estimated gas costs. Make sure you look at the exchange rate for your costs in Europe.
Your trip sounds very exciting! Great plan to get away before those babies come!