Don't.
We have one, and we adore her. But she has an Anti-G disorder that's a known problem with the breed so practically no immune system. If she gets a bug she has to go on three sorts of drugs - gut soothers, pain relievers, and antibiotics. For three months each time. She's also on a prescription only diet after much trial and error. You might also want to google Chiari Malformation and Syringa Myelia, as 95% now have the first, which often leads to the second.
The ver hospital we have to go to annually in Cambridge (it's attached to the uni and one of the top 2 in the country) also worried that she might have a liver shunt, another very serious problem not uncommon in Cavvies. They told us that they almost died out in the 1920s and were then bred up from about 20 dogs. The inbreeding is appalling, even without the irresponsible breeding for appearance everyone knows about. They are lovely, lovely dogs, and we adore ours, but the vet hospital said there are other breeds they can recommend that are far kinder to the animal than encouraging the creation of very fragile lives. Alison Collings is lovely there and has treated Izzy all her adult life; she will have a chat with you as a fellow dog owner if you are interested, I'm sure.
If you really do want to go ahead regardless, for the love of God, get Petplan insurance. You still have to pay 90 quid a pop that isn't claimable, and Cavvies are 29 quid a month to insure, but given she averages about 4 grand a year in vets bills, it was the best decision ever.
Also second the reputable breeder. You need a specialist - not a puppy farm, but not a family who just fancy puppies, either. They need to really know their stuff... although as mentioned, as 95% now have the CM, it's pretty much unavoidable and a very nasty condition if it develops into the SM.