Hey! I'm a Cav person!
More than happy to help you out here. There's also a Cav-specific board that has lots of very knowledgeable people.
It's at http://www.ckcsboard.com/messageboard
(hope it's ok to post that...?)
MVD... ok, here's the deal. It will basically affect the dog at some point in its life. Pretty much all Cavs get MVD. 50% of them will have it by age 5. It is, however, a manageable disease. At least, it can be. Dogs can live for YEARS with the condition (with proper care) if it doesn't progress too quickly.
To avoid the dog getting MVD early in life, you MUST GET YOUR DOG FROM A REPUTABLE BREEDER. The mother and father should BOTH be heart clear, and preferably be over age 5 (this increases the chances of you having a healthy pup that STAYS healthy for a longer amount of time). Thing is the testing is expensive, so if you want a Cav, be prepared to pay a lot of money for them. PLEASE do not go the cheap way and try to get a cheaper pup. You also need to make sure you can see PROOF of the heart clearances, as plenty of "breeders" can say they test their dogs and just never show you papers.
They are worth every penny though, they are the sweetest things in the world. (if you get them from a good breeder, of course)
A GREAT site to get information from is the Premier Cavalier Infosite... the address for that is: http://premiercavalierinfosite.com/
Sorry to keep rambling... if you have ANY more questions feel free to ask.
I LOVE answering questions about Cavs.
I woul dhelp you with the English Toy Spaniel bit but I honestly know nohting about them... sorry!
I'm trying ot think if htere's anything else in particular you need to know... not much I can think of right now. You should definitely check out the above listed site if you haven't seen it already.
Let us know how it goes with getting your dog! My Cavalier is in my signature.
And like I said, if you have ANY questions, I'm MORE than happy to answer them!