I have seen a 1998 Lantra for sale in good condition. The only thing is that the seller only has two black keys; no blue or purple master key. Is this going to be a problem? I know I can get the black keys cloned by an auto locksmith for around £25 but am not sure if the ecu ever gets itself into a state where it will only recognise the master.
Would you advise avoiding a car without the master key ?

the master key enables the dealer to programme other keys into the system (there won't be a problem as long as you don't loose your keys)

I've never had a master key with my Elantra since we bought it used and it came from auction. Even the "original" key they gave me was a copy. I just made a few more copies and use those with no problem.

My understanding is that the blue 'master' key and the ecu are a matched pair. That means that the ecu will always recognise the master key. The other black keys can be programmed in BUT the information is held in flash RAM (whereas the Blue key details are held in ROM). This means if the flash RAM ever gets erased by a voltage spike, or when connecting the car battery or by some other unfortunate occurance, then you are faced with replacing the ecu, immobiliser, and transponder arial (around the ignition lock). As I was quoted about £1,400 from Hyundai for that little lot, it basically means the car would be a write-off without the Blue key if the ecu ever 'forgets' the other information. I know this is unlikely but it has happened to some Fiat owners to their cost.
This probably only applies to UK cars where immobilisers have been compulsory since 1997 to cut car theft.
I have now found a Lantra which has the master key so bought that one instead. A 1999 V reg with 100K for £500.

Interesting. I guess the cars are different in the U.S. since I've never had a master key and I've disconnected the battery plenty of times and reset the ecu and never had any problems