I am a Single ol lady and have been on 32 cruises, mostly with Princess. One on Epic (NCL) and one on RCCL (Oasis of the Seas).
You will find that Europe is not as friendly to people who have difficulty walking. There are uneven cobblestones (look darling but hard on the back and legs). Not all countries have the corners that slant down.
I use my Cane-Seat (I call it). Magellan's calls it a Sports Seat. I use it, mainly, to sit on when stopping with a tour guide or whenever U must wait in line, like at the airport or the Purser's Desk on board. It is also handy under my arm, while walking, to hold like a Tommie Gun. Also, I look people approaching me right in the eye and U do not look away until they do. I shall practice this skill in June in Barcelona - the 2nd most pick-pocketed city in the world.
I don't know what cruise line you are looking at; however, I advise to book early on any. Send for brochures from a lot of them (they ARE free) and come within a few days. Look at the Deck Plans. Handicapped cabins have a mark on them (usually an open o). These cabins have wider doors, as well as larger balconies and roll-in (much larger) showers. Cost is the same.
The 3 of you may want to get a mini-suite; however, I don't know if there are any that are handicapped.
On the cruise lines' websites, you can see photos and room layouts. I know that on the Princess website, you can read detailed descriptions of the Shore Excursions for different ports-of-call. They are marked with level of difficulty and some are designed especially for wheelchair passengers. Some of them you can just drive around on the bus and never get off, or get off and walk in town - your choice.
Most countries (it doesn't go by Cities) in Europe use the Euro. Look at www.oanda.com
for conversion rates and get currency from your bank before leaving. I would invest about $10 - $15 in a silk wallet. I wear it under my panties and put all my currencies in it to fly and then secure it in my cabin safe on board. I take only one credit card but use it only to pay my Folio on board at the end of the cruise. Look at TravelSmith on line or Magellans.
Most of the British Isles (Wales, Ireland, Scotland, England) use the GBP (Great British Pound). As far as how much cash you will need, it depends on how much you like to shop and how many friends & family need souvenirs. For me, $2000 USD for food ashore, souvenirs, tips to tour guides, cab fare, etc. I am now figuring out my budget for June. My pre-cruise and post-cruise B&Bs in London and Barcelona are already paid for. Don't forget that the cruise line will add about $10 to $15 per person per day for gratuities. That way you can put it into your budget and even pre-pay it on line. Those tips cover everyone from your Cabin Steward, Waiter, laundry person, etc. Barmen add 15% more on for any drinks.
You do not have to eat in the extra-pay restaurants. I never do and the food is always wonderful and I don't have to cook! You don't have to drink, although you can purchase Drink Packages. I save my vitamin bottles and take different Liqueurs. Put into plastic baggies, put that into a sock, put the sock in a shoe, and there U go.
Yes, take your laptop. In some ports you can use it free or find an inexpensive cyber cafe. It IS very costly on board. I am Elite on Princess so I get a couple of hours free. You should compose your e-mails off line and then it takes very little time to Send. I also Journal every day and my family really enjoys this.
Once you have booked, join your Roll Call on Cruise Critic and your fellow pax will be - oh-so-helpful. Sometimes, they will plan private tours in the different ports. Most of the time, I do this. However, for your very first cruise, U may want to stick with the ship-sponsored shore.
I hope I have been some help to you. There is nothing like cruising and it is really time that You get to it.